diff --git a/internal/gensupport/json.go b/internal/gensupport/json.go
index 1b770464110..eab49a11eb1 100644
--- a/internal/gensupport/json.go
+++ b/internal/gensupport/json.go
@@ -86,7 +86,12 @@ func schemaToMap(schema interface{}, mustInclude, useNull map[string]bool, useNu
 		if f.Type.Kind() == reflect.Map && useNullMaps[f.Name] != nil {
 			ms, ok := v.Interface().(map[string]string)
 			if !ok {
-				return nil, fmt.Errorf("field %q has keys in NullFields but is not a map[string]string", f.Name)
+				mi, err := initMapSlow(v, f.Name, useNullMaps)
+				if err != nil {
+					return nil, err
+				}
+				m[tag.apiName] = mi
+				continue
 			}
 			mi := map[string]interface{}{}
 			for k, v := range ms {
@@ -120,6 +125,25 @@ func schemaToMap(schema interface{}, mustInclude, useNull map[string]bool, useNu
 	return m, nil
 }
 
+// initMapSlow uses reflection to build up a map object. This is slower than
+// the default behavior so it should be used only as a fallback.
+func initMapSlow(rv reflect.Value, fieldName string, useNullMaps map[string]map[string]bool) (map[string]interface{}, error) {
+	mi := map[string]interface{}{}
+	iter := rv.MapRange()
+	for iter.Next() {
+		k, ok := iter.Key().Interface().(string)
+		if !ok {
+			return nil, fmt.Errorf("field %q has keys in NullFields but is not a map[string]any", fieldName)
+		}
+		v := iter.Value().Interface()
+		mi[k] = v
+	}
+	for k := range useNullMaps[fieldName] {
+		mi[k] = nil
+	}
+	return mi, nil
+}
+
 // formatAsString returns a string representation of v, dereferencing it first if possible.
 func formatAsString(v reflect.Value, kind reflect.Kind) string {
 	if kind == reflect.Ptr && !v.IsNil() {
diff --git a/internal/gensupport/json_test.go b/internal/gensupport/json_test.go
index 2cc5dfbbb90..d5ae3a9a14b 100644
--- a/internal/gensupport/json_test.go
+++ b/internal/gensupport/json_test.go
@@ -12,6 +12,10 @@ import (
 	"google.golang.org/api/googleapi"
 )
 
+type CustomType struct {
+	Foo string `json:"foo,omitempty"`
+}
+
 type schema struct {
 	// Basic types
 	B    bool    `json:"b,omitempty"`
@@ -28,12 +32,13 @@ type schema struct {
 	PStr  *string  `json:"pstr,omitempty"`
 
 	// Other types
-	Int64s        googleapi.Int64s         `json:"i64s,omitempty"`
-	S             []int                    `json:"s,omitempty"`
-	M             map[string]string        `json:"m,omitempty"`
-	Any           interface{}              `json:"any,omitempty"`
-	Child         *child                   `json:"child,omitempty"`
-	MapToAnyArray map[string][]interface{} `json:"maptoanyarray,omitempty"`
+	Int64s          googleapi.Int64s         `json:"i64s,omitempty"`
+	S               []int                    `json:"s,omitempty"`
+	M               map[string]string        `json:"m,omitempty"`
+	Any             interface{}              `json:"any,omitempty"`
+	Child           *child                   `json:"child,omitempty"`
+	MapToAnyArray   map[string][]interface{} `json:"maptoanyarray,omitempty"`
+	MapToCustomType map[string]CustomType    `json:"maptocustomtype,omitempty"`
 
 	ForceSendFields []string `json:"-"`
 	NullFields      []string `json:"-"`
@@ -254,6 +259,15 @@ func TestMapField(t *testing.T) {
 			},
 			want: `{}`,
 		},
+		{
+			s: schema{
+				MapToCustomType: map[string]CustomType{
+					"a": {Foo: "foo"},
+				},
+				NullFields: []string{"MapToCustomType.b"},
+			},
+			want: `{"maptocustomtype": {"a": {"foo": "foo"}, "b": null}}`,
+		},
 	} {
 		checkMarshalJSON(t, tc)
 	}