diff --git a/issue66_test.go b/issue66_test.go
index 9e4bcce..ebc4276 100644
--- a/issue66_test.go
+++ b/issue66_test.go
@@ -21,7 +21,7 @@ func TestPrivateSlice(t *testing.T) {
 		t.Fatalf("Error during the merge: %v", err)
 	}
 	if len(p1.PublicStrings) != 3 {
-		t.Error("5 elements should be in 'PublicStrings' field")
+		t.Error("3 elements should be in 'PublicStrings' field, when no append")
 	}
 	if len(p1.privateStrings) != 2 {
 		t.Error("2 elements should be in 'privateStrings' field")
diff --git a/issue90_test.go b/issue90_test.go
new file mode 100644
index 0000000..311bf78
--- /dev/null
+++ b/issue90_test.go
@@ -0,0 +1,50 @@
+package mergo
+
+import (
+	"reflect"
+	"testing"
+)
+
+type CustomStruct struct {
+	SomeMap map[string]string
+}
+
+func TestMergoStructMap(t *testing.T) {
+	var testData = []struct {
+		name string
+		src  map[string]CustomStruct
+		dst  map[string]CustomStruct
+		exp  map[string]CustomStruct
+	}{
+		{name: "Normal",
+			dst: map[string]CustomStruct{"a": CustomStruct{SomeMap: map[string]string{"key1": "loosethis", "key2": "keepthis"}}},
+			src: map[string]CustomStruct{"a": CustomStruct{SomeMap: map[string]string{"key1": "key10"}}},
+			exp: map[string]CustomStruct{"a": CustomStruct{SomeMap: map[string]string{"key1": "key10", "key2": "keepthis"}}},
+		},
+		{name: "Init of struct key", dst: map[string]CustomStruct{"a": CustomStruct{SomeMap: map[string]string{}}},
+			src: map[string]CustomStruct{"a": CustomStruct{SomeMap: map[string]string{"key1": "key10"}}},
+			exp: map[string]CustomStruct{"a": CustomStruct{SomeMap: map[string]string{"key1": "key10"}}},
+		},
+		{name: "Not Init of struct key", dst: map[string]CustomStruct{},
+			src: map[string]CustomStruct{"a": CustomStruct{SomeMap: map[string]string{"key1": "key10"}}},
+			exp: map[string]CustomStruct{"a": CustomStruct{SomeMap: map[string]string{"key1": "key10"}}},
+		},
+		{name: "Nil struct key", dst: map[string]CustomStruct{"a": CustomStruct{SomeMap: nil}},
+			src: map[string]CustomStruct{"a": CustomStruct{SomeMap: map[string]string{"key1": "key10"}}},
+			exp: map[string]CustomStruct{"a": CustomStruct{SomeMap: map[string]string{"key1": "key10"}}}},
+	}
+
+	for _, data := range testData {
+		dst := data.dst
+		src := data.src
+		exp := data.exp
+
+		err := Merge(&dst, src, WithAppendSlice, WithOverride)
+		if err != nil {
+			t.Errorf("mergo error was not nil, %v", err)
+		}
+		if !reflect.DeepEqual(dst, exp) {
+			t.Errorf("Actual: %#v did not match \nExpected: %#v", dst, exp)
+		}
+	}
+}
diff --git a/map.go b/map.go
index 3f5afa8..d83258b 100644
--- a/map.go
+++ b/map.go
@@ -99,11 +99,11 @@ func deepMap(dst, src reflect.Value, visited map[uintptr]*visit, depth int, conf
 				continue
 			}
 			if srcKind == dstKind {
-				if err = deepMerge(dstElement, srcElement, visited, depth+1, config); err != nil {
+				if _, err = deepMerge(dstElement, srcElement, visited, depth+1, config); err != nil {
 					return
 				}
 			} else if dstKind == reflect.Interface && dstElement.Kind() == reflect.Interface {
-				if err = deepMerge(dstElement, srcElement, visited, depth+1, config); err != nil {
+				if _, err = deepMerge(dstElement, srcElement, visited, depth+1, config); err != nil {
 					return
 				}
 			} else if srcKind == reflect.Map {
@@ -157,7 +157,8 @@ func _map(dst, src interface{}, opts ...func(*Config)) error {
 	// To be friction-less, we redirect equal-type arguments
 	// to deepMerge. Only because arguments can be anything.
 	if vSrc.Kind() == vDst.Kind() {
-		return deepMerge(vDst, vSrc, make(map[uintptr]*visit), 0, config)
+		_, err := deepMerge(vDst, vSrc, make(map[uintptr]*visit), 0, config)
+		return err
 	}
 	switch vSrc.Kind() {
 	case reflect.Struct:
diff --git a/merge.go b/merge.go
index 87eb70c..3332c9c 100644
--- a/merge.go
+++ b/merge.go
@@ -11,20 +11,32 @@ package mergo
 import (
 	"fmt"
 	"reflect"
+	"unsafe"
 )
 
 func hasExportedField(dst reflect.Value) (exported bool) {
 	for i, n := 0, dst.NumField(); i < n; i++ {
 		field := dst.Type().Field(i)
-		if field.Anonymous && dst.Field(i).Kind() == reflect.Struct {
-			exported = exported || hasExportedField(dst.Field(i))
-		} else {
-			exported = exported || len(field.PkgPath) == 0
+		if isExportedComponent(&field) {
+			return true
 		}
 	}
 	return
 }
 
+func isExportedComponent(field *reflect.StructField) bool {
+	name := field.Name
+	pkgPath := field.PkgPath
+	if len(pkgPath) > 0 {
+		return false
+	}
+	c := name[0]
+	if 'a' <= c && c <= 'z' || c == '_' {
+		return false
+	}
+	return true
+}
+
 type Config struct {
 	Overwrite                    bool
 	AppendSlice                  bool
@@ -41,7 +53,8 @@ type Transformers interface {
 // Traverses recursively both values, assigning src's fields values to dst.
 // The map argument tracks comparisons that have already been seen, which allows
 // short circuiting on recursive types.
-func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, config *Config) (err error) {
+func deepMerge(dstIn, src reflect.Value, visited map[uintptr]*visit, depth int, config *Config) (dst reflect.Value, err error) {
+	dst = dstIn
 	overwrite := config.Overwrite
 	typeCheck := config.TypeCheck
 	overwriteWithEmptySrc := config.overwriteWithEmptyValue
@@ -50,6 +63,7 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 	if !src.IsValid() {
 		return
 	}
+
 	if dst.CanAddr() {
 		addr := dst.UnsafeAddr()
 		h := 17 * addr
@@ -57,7 +71,7 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 		typ := dst.Type()
 		for p := seen; p != nil; p = p.next {
 			if p.ptr == addr && p.typ == typ {
-				return nil
+				return dst, nil
 			}
 		}
 		// Remember, remember...
@@ -71,114 +85,124 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 		}
 	}
 
+	if dst.IsValid() && src.IsValid() && src.Type() != dst.Type() {
+		err = fmt.Errorf("cannot append two different types (%s, %s)", src.Kind(), dst.Kind())
+		return
+	}
+
 	switch dst.Kind() {
 	case reflect.Struct:
 		if hasExportedField(dst) {
+			dstCp := reflect.New(dst.Type()).Elem()
 			for i, n := 0, dst.NumField(); i < n; i++ {
-				if err = deepMerge(dst.Field(i), src.Field(i), visited, depth+1, config); err != nil {
+				dstField := dst.Field(i)
+				structField := dst.Type().Field(i)
+				// copy un-exported struct fields
+				if !isExportedComponent(&structField) {
+					rf := dstCp.Field(i)
+					rf = reflect.NewAt(rf.Type(), unsafe.Pointer(rf.UnsafeAddr())).Elem() //nolint:gosec
+					dstRF := dst.Field(i)
+					if !dst.Field(i).CanAddr() {
+						continue
+					}
+
+					dstRF = reflect.NewAt(dstRF.Type(), unsafe.Pointer(dstRF.UnsafeAddr())).Elem() //nolint:gosec
+					rf.Set(dstRF)
+					continue
+				}
+				dstField, err = deepMerge(dstField, src.Field(i), visited, depth+1, config)
+				if err != nil {
 					return
 				}
+				dstCp.Field(i).Set(dstField)
 			}
+
+			if dst.CanSet() {
+				dst.Set(dstCp)
+			} else {
+				dst = dstCp
+			}
+			return
 		} else {
-			if dst.CanSet() && (!isEmptyValue(src) || overwriteWithEmptySrc) && (overwrite || isEmptyValue(dst)) {
-				dst.Set(src)
+			if (isReflectNil(dst) || overwrite) && (!isEmptyValue(src) || overwriteWithEmptySrc) {
+				dst = src
 			}
 		}
+
 	case reflect.Map:
 		if dst.IsNil() && !src.IsNil() {
-			dst.Set(reflect.MakeMap(dst.Type()))
+			if dst.CanSet() {
+				dst.Set(reflect.MakeMap(dst.Type()))
+			} else {
+				dst = src
+				return
+			}
 		}
 		for _, key := range src.MapKeys() {
 			srcElement := src.MapIndex(key)
+			dstElement := dst.MapIndex(key)
 			if !srcElement.IsValid() {
 				continue
 			}
-			dstElement := dst.MapIndex(key)
-			switch srcElement.Kind() {
-			case reflect.Chan, reflect.Func, reflect.Map, reflect.Interface, reflect.Slice:
-				if srcElement.IsNil() {
-					continue
-				}
-				fallthrough
-			default:
-				if !srcElement.CanInterface() {
-					continue
-				}
-				switch reflect.TypeOf(srcElement.Interface()).Kind() {
-				case reflect.Struct:
-					fallthrough
-				case reflect.Ptr:
-					fallthrough
-				case reflect.Map:
-					srcMapElm := srcElement
-					dstMapElm := dstElement
-					if srcMapElm.CanInterface() {
-						srcMapElm = reflect.ValueOf(srcMapElm.Interface())
-						if dstMapElm.IsValid() {
-							dstMapElm = reflect.ValueOf(dstMapElm.Interface())
-						}
-					}
-					if err = deepMerge(dstMapElm, srcMapElm, visited, depth+1, config); err != nil {
-						return
-					}
-				case reflect.Slice:
-					srcSlice := reflect.ValueOf(srcElement.Interface())
-
-					var dstSlice reflect.Value
-					if !dstElement.IsValid() || dstElement.IsNil() {
-						dstSlice = reflect.MakeSlice(srcSlice.Type(), 0, srcSlice.Len())
-					} else {
-						dstSlice = reflect.ValueOf(dstElement.Interface())
-					}
-
-					if (!isEmptyValue(src) || overwriteWithEmptySrc || overwriteSliceWithEmptySrc) && (overwrite || isEmptyValue(dst)) && !config.AppendSlice {
-						if typeCheck && srcSlice.Type() != dstSlice.Type() {
-							return fmt.Errorf("cannot override two slices with different type (%s, %s)", srcSlice.Type(), dstSlice.Type())
-						}
-						dstSlice = srcSlice
-					} else if config.AppendSlice {
-						if srcSlice.Type() != dstSlice.Type() {
-							return fmt.Errorf("cannot append two slices with different type (%s, %s)", srcSlice.Type(), dstSlice.Type())
-						}
-						dstSlice = reflect.AppendSlice(dstSlice, srcSlice)
-					}
-					dst.SetMapIndex(key, dstSlice)
+			if dst.MapIndex(key).IsValid() {
+				k := dstElement.Interface()
+				dstElement = reflect.ValueOf(k)
+			}
+			if isReflectNil(srcElement) {
+				if overwrite || isReflectNil(dstElement) {
+					dst.SetMapIndex(key, srcElement)
 				}
+				continue
 			}
-			if dstElement.IsValid() && !isEmptyValue(dstElement) && (reflect.TypeOf(srcElement.Interface()).Kind() == reflect.Map || reflect.TypeOf(srcElement.Interface()).Kind() == reflect.Slice) {
+			if !srcElement.CanInterface() {
 				continue
 			}
 
-			if srcElement.IsValid() && ((srcElement.Kind() != reflect.Ptr && overwrite) || !dstElement.IsValid() || isEmptyValue(dstElement)) {
-				if dst.IsNil() {
-					dst.Set(reflect.MakeMap(dst.Type()))
+			if srcElement.CanInterface() {
+				srcElement = reflect.ValueOf(srcElement.Interface())
+				if dstElement.IsValid() {
+					dstElement = reflect.ValueOf(dstElement.Interface())
 				}
-				dst.SetMapIndex(key, srcElement)
 			}
+			dstElement, err = deepMerge(dstElement, srcElement, visited, depth+1, config)
+			if err != nil {
+				return
+			}
+			dst.SetMapIndex(key, dstElement)
+
 		}
 	case reflect.Slice:
-		if !dst.CanSet() {
-			break
-		}
+		newSlice := dst
 		if (!isEmptyValue(src) || overwriteWithEmptySrc || overwriteSliceWithEmptySrc) && (overwrite || isEmptyValue(dst)) && !config.AppendSlice {
-			dst.Set(src)
+			if typeCheck && src.Type() != dst.Type() {
+				return dst, fmt.Errorf("cannot override two slices with different type (%s, %s)", src.Type(), dst.Type())
+			}
+			newSlice = src
 		} else if config.AppendSlice {
-			if src.Type() != dst.Type() {
-				return fmt.Errorf("cannot append two slice with different type (%s, %s)", src.Type(), dst.Type())
+			if typeCheck && src.Type() != dst.Type() {
+				err = fmt.Errorf("cannot append two slice with different type (%s, %s)", src.Type(), dst.Type())
+				return
 			}
-			dst.Set(reflect.AppendSlice(dst, src))
+			newSlice = reflect.AppendSlice(dst, src)
 		}
-	case reflect.Ptr:
-		fallthrough
-	case reflect.Interface:
-		if src.IsNil() {
+		if dst.CanSet() {
+			dst.Set(newSlice)
+		} else {
+			dst = newSlice
+		}
+	case reflect.Ptr, reflect.Interface:
+		if isReflectNil(src) {
 			break
 		}
 
 		if dst.Kind() != reflect.Ptr && src.Type().AssignableTo(dst.Type()) {
 			if dst.IsNil() || overwrite {
-				if dst.CanSet() && (overwrite || isEmptyValue(dst)) {
-					dst.Set(src)
+				if overwrite || isEmptyValue(dst) {
+					if dst.CanSet() {
+						dst.Set(src)
+					} else {
+						dst = src
+					}
 				}
 			}
 			break
@@ -190,28 +214,38 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 					dst.Set(src)
 				}
 			} else if src.Kind() == reflect.Ptr {
-				if err = deepMerge(dst.Elem(), src.Elem(), visited, depth+1, config); err != nil {
+				if dst, err = deepMerge(dst.Elem(), src.Elem(), visited, depth+1, config); err != nil {
 					return
 				}
+				dst = dst.Addr()
 			} else if dst.Elem().Type() == src.Type() {
-				if err = deepMerge(dst.Elem(), src, visited, depth+1, config); err != nil {
+				if dst, err = deepMerge(dst.Elem(), src, visited, depth+1, config); err != nil {
 					return
 				}
 			} else {
-				return ErrDifferentArgumentsTypes
+				return dst, ErrDifferentArgumentsTypes
 			}
 			break
 		}
 		if dst.IsNil() || overwrite {
-			if dst.CanSet() && (overwrite || isEmptyValue(dst)) {
-				dst.Set(src)
+			if (overwrite || isEmptyValue(dst)) && (overwriteWithEmptySrc || !isEmptyValue(src)) {
+				if dst.CanSet() {
+					dst.Set(src)
+				} else {
+					dst = src
+				}
 			}
-		} else if err = deepMerge(dst.Elem(), src.Elem(), visited, depth+1, config); err != nil {
+		} else if _, err = deepMerge(dst.Elem(), src.Elem(), visited, depth+1, config); err != nil {
 			return
 		}
 	default:
-		if dst.CanSet() && (!isEmptyValue(src) || overwriteWithEmptySrc) && (overwrite || isEmptyValue(dst)) {
-			dst.Set(src)
+		overwriteFull := (!isEmptyValue(src) || overwriteWithEmptySrc) && (overwrite || isEmptyValue(dst))
+		if overwriteFull {
+			if dst.CanSet() {
+				dst.Set(src)
+			} else {
+				dst = src
+			}
 		}
 	}
 
@@ -280,8 +314,25 @@ func merge(dst, src interface{}, opts ...func(*Config)) error {
 	if vDst, vSrc, err = resolveValues(dst, src); err != nil {
 		return err
 	}
+	if !vDst.CanSet() {
+		return fmt.Errorf("cannot set dst, needs reference")
+	}
 	if vDst.Type() != vSrc.Type() {
 		return ErrDifferentArgumentsTypes
 	}
-	return deepMerge(vDst, vSrc, make(map[uintptr]*visit), 0, config)
+	_, err = deepMerge(vDst, vSrc, make(map[uintptr]*visit), 0, config)
+	return err
+}
+
+// IsReflectNil is the reflect value provided nil
+func isReflectNil(v reflect.Value) bool {
+	k := v.Kind()
+	switch k {
+	case reflect.Interface, reflect.Slice, reflect.Chan, reflect.Func, reflect.Map, reflect.Ptr:
+		// Both interface and slice are nil if first word is 0.
+		// Both are always bigger than a word; assume flagIndir.
+		return v.IsNil()
+	default:
+		return false
+	}
 }
diff --git a/mergo_test.go b/mergo_test.go
index 1786824..cb28e4d 100644
--- a/mergo_test.go
+++ b/mergo_test.go
@@ -129,10 +129,10 @@ func TestComplexStruct(t *testing.T) {
 }
 
 func TestComplexStructWithOverwrite(t *testing.T) {
-	a := complexTest{simpleTest{1}, 1, "do-not-overwrite-with-empty-value"}
-	b := complexTest{simpleTest{42}, 2, ""}
+	a := complexTest{St: simpleTest{1}, sz: 1, ID: "do-not-overwrite-with-empty-value"}
+	b := complexTest{St: simpleTest{42}, sz: 2, ID: ""}
 
-	expect := complexTest{simpleTest{42}, 1, "do-not-overwrite-with-empty-value"}
+	expect := complexTest{St: simpleTest{42}, sz: 1, ID: "do-not-overwrite-with-empty-value"}
 	if err := MergeWithOverwrite(&a, b); err != nil {
 		t.FailNow()
 	}
@@ -156,7 +156,7 @@ func TestPointerStruct(t *testing.T) {
 }
 
 type embeddingStruct struct {
-	embeddedStruct
+	A embeddedStruct
 }
 
 type embeddedStruct struct {
@@ -345,13 +345,13 @@ func TestEmptyToNotEmptyMaps(t *testing.T) {
 }
 
 func TestMapsWithOverwrite(t *testing.T) {
-	m := map[string]simpleTest{
+	dst := map[string]simpleTest{
 		"a": {},   // overwritten by 16
 		"b": {42}, // overwritten by 0, as map Value is not addressable and it doesn't check for b is set or not set in `n`
 		"c": {13}, // overwritten by 12
 		"d": {61},
 	}
-	n := map[string]simpleTest{
+	src := map[string]simpleTest{
 		"a": {16},
 		"b": {},
 		"c": {12},
@@ -359,18 +359,18 @@ func TestMapsWithOverwrite(t *testing.T) {
 	}
 	expect := map[string]simpleTest{
 		"a": {16},
-		"b": {},
+		"b": {42},
 		"c": {12},
 		"d": {61},
 		"e": {14},
 	}
 
-	if err := MergeWithOverwrite(&m, n); err != nil {
+	if err := MergeWithOverwrite(&dst, src); err != nil {
 		t.Fatalf(err.Error())
 	}
 
-	if !reflect.DeepEqual(m, expect) {
-		t.Fatalf("Test failed:\ngot  :\n%#v\n\nwant :\n%#v\n\n", m, expect)
+	if !reflect.DeepEqual(dst, expect) {
+		t.Fatalf("Test failed:\ngot  :\n%#v\n\nwant :\n%#v\n\n", dst, expect)
 	}
 }
 
@@ -536,23 +536,13 @@ func TestMergeUsingStructAndMap(t *testing.T) {
 }
 func TestMaps(t *testing.T) {
 	m := map[string]simpleTest{
-		"a": {},
-		"b": {42},
-		"c": {13},
-		"d": {61},
+		"a": {0}, "b": {42}, "c": {13}, "d": {61},
 	}
 	n := map[string]simpleTest{
-		"a": {16},
-		"b": {},
-		"c": {12},
-		"e": {14},
+		"a": {16}, "b": {}, "c": {12}, "e": {14},
 	}
 	expect := map[string]simpleTest{
-		"a": {0},
-		"b": {42},
-		"c": {13},
-		"d": {61},
-		"e": {14},
+		"a": {16}, "b": {42}, "c": {13}, "d": {61}, "e": {14},
 	}
 
 	if err := Merge(&m, n); err != nil {
@@ -562,7 +552,7 @@ func TestMaps(t *testing.T) {
 	if !reflect.DeepEqual(m, expect) {
 		t.Fatalf("Test failed:\ngot  :\n%#v\n\nwant :\n%#v\n\n", m, expect)
 	}
-	if m["a"].Value != 0 {
+	if m["a"].Value != 16 {
 		t.Fatalf(`n merged in m because I solved non-addressable map values TODO: m["a"].Value(%d) != n["a"].Value(%d)`, m["a"].Value, n["a"].Value)
 	}
 	if m["b"].Value != 42 {
@@ -903,12 +893,12 @@ func TestMergeMapWithInnerSliceOfDifferentType(t *testing.T) {
 		{
 			"With override and append slice",
 			[]func(*Config){WithOverride, WithAppendSlice},
-			"cannot append two slices with different type",
+			"cannot append two different types (slice, slice)",
 		},
 		{
 			"With override and type check",
 			[]func(*Config){WithOverride, WithTypeCheck},
-			"cannot override two slices with different type",
+			"cannot append two different types (slice, slice)",
 		},
 	}
 	for _, tc := range testCases {