diff --git a/internal/terraform/eval_for_each.go b/internal/terraform/eval_for_each.go
index fccf58f915b0..ba7a8c9bca6c 100644
--- a/internal/terraform/eval_for_each.go
+++ b/internal/terraform/eval_for_each.go
@@ -78,6 +78,9 @@ func evaluateForEachExpressionValue(expr hcl.Expression, ctx EvalContext, allowU
 	}
 	ty := forEachVal.Type()
 
+	const errInvalidUnknownDetailMap = "The \"for_each\" map includes keys derived from resource attributes that cannot be determined until apply, and so Terraform cannot determine the full set of keys that will identify the instances of this resource.\n\nWhen working with unknown values in for_each, it's better to define the map keys statically in your configuration and place apply-time results only in the map values.\n\nAlternatively, you could use the -target planning option to first apply only the resources that the for_each value depends on, and then apply a second time to fully converge."
+	const errInvalidUnknownDetailSet = "The \"for_each\" set includes values derived from resource attributes that cannot be determined until apply, and so Terraform cannot determine the full set of keys that will identify the instances of this resource.\n\nWhen working with unknown values in for_each, it's better to use a map value where the keys are defined statically in your configuration and where only the values contain apply-time results.\n\nAlternatively, you could use the -target planning option to first apply only the resources that the for_each value depends on, and then apply a second time to fully converge."
+
 	switch {
 	case forEachVal.IsNull():
 		diags = diags.Append(&hcl.Diagnostic{
@@ -91,10 +94,18 @@ func evaluateForEachExpressionValue(expr hcl.Expression, ctx EvalContext, allowU
 		return nullMap, diags
 	case !forEachVal.IsKnown():
 		if !allowUnknown {
+			var detailMsg string
+			switch {
+			case ty.IsSetType():
+				detailMsg = errInvalidUnknownDetailSet
+			default:
+				detailMsg = errInvalidUnknownDetailMap
+			}
+
 			diags = diags.Append(&hcl.Diagnostic{
 				Severity:    hcl.DiagError,
 				Summary:     "Invalid for_each argument",
-				Detail:      errInvalidForEachUnknownDetail,
+				Detail:      detailMsg,
 				Subject:     expr.Range().Ptr(),
 				Expression:  expr,
 				EvalContext: hclCtx,
@@ -129,7 +140,7 @@ func evaluateForEachExpressionValue(expr hcl.Expression, ctx EvalContext, allowU
 				diags = diags.Append(&hcl.Diagnostic{
 					Severity:    hcl.DiagError,
 					Summary:     "Invalid for_each argument",
-					Detail:      errInvalidForEachUnknownDetail,
+					Detail:      errInvalidUnknownDetailSet,
 					Subject:     expr.Range().Ptr(),
 					Expression:  expr,
 					EvalContext: hclCtx,
@@ -172,8 +183,6 @@ func evaluateForEachExpressionValue(expr hcl.Expression, ctx EvalContext, allowU
 	return forEachVal, nil
 }
 
-const errInvalidForEachUnknownDetail = `The "for_each" value depends on resource attributes that cannot be determined until apply, so Terraform cannot predict how many instances will be created. To work around this, use the -target argument to first apply only the resources that the for_each depends on.`
-
 // markSafeLengthInt allows calling LengthInt on marked values safely
 func markSafeLengthInt(val cty.Value) int {
 	v, _ := val.UnmarkDeep()
diff --git a/internal/terraform/eval_for_each_test.go b/internal/terraform/eval_for_each_test.go
index 28a025ab0d47..be4b551cf039 100644
--- a/internal/terraform/eval_for_each_test.go
+++ b/internal/terraform/eval_for_each_test.go
@@ -114,12 +114,12 @@ func TestEvaluateForEachExpression_errors(t *testing.T) {
 		"unknown string set": {
 			hcltest.MockExprLiteral(cty.UnknownVal(cty.Set(cty.String))),
 			"Invalid for_each argument",
-			"depends on resource attributes that cannot be determined until apply",
+			"set includes values derived from resource attributes that cannot be determined until apply",
 		},
 		"unknown map": {
 			hcltest.MockExprLiteral(cty.UnknownVal(cty.Map(cty.Bool))),
 			"Invalid for_each argument",
-			"depends on resource attributes that cannot be determined until apply",
+			"map includes keys derived from resource attributes that cannot be determined until apply",
 		},
 		"marked map": {
 			hcltest.MockExprLiteral(cty.MapVal(map[string]cty.Value{
@@ -142,12 +142,12 @@ func TestEvaluateForEachExpression_errors(t *testing.T) {
 		"set containing unknown value": {
 			hcltest.MockExprLiteral(cty.SetVal([]cty.Value{cty.UnknownVal(cty.String)})),
 			"Invalid for_each argument",
-			"depends on resource attributes that cannot be determined until apply",
+			"set includes values derived from resource attributes that cannot be determined until apply",
 		},
 		"set containing dynamic unknown value": {
 			hcltest.MockExprLiteral(cty.SetVal([]cty.Value{cty.UnknownVal(cty.DynamicPseudoType)})),
 			"Invalid for_each argument",
-			"depends on resource attributes that cannot be determined until apply",
+			"set includes values derived from resource attributes that cannot be determined until apply",
 		},
 		"set containing marked values": {
 			hcltest.MockExprLiteral(cty.SetVal([]cty.Value{cty.StringVal("beep").Mark(marks.Sensitive), cty.StringVal("boop")})),
@@ -169,10 +169,10 @@ func TestEvaluateForEachExpression_errors(t *testing.T) {
 				t.Errorf("wrong diagnostic severity %#v; want %#v", got, want)
 			}
 			if got, want := diags[0].Description().Summary, test.Summary; got != want {
-				t.Errorf("wrong diagnostic summary %#v; want %#v", got, want)
+				t.Errorf("wrong diagnostic summary\ngot:  %s\nwant: %s", got, want)
 			}
 			if got, want := diags[0].Description().Detail, test.DetailSubstring; !strings.Contains(got, want) {
-				t.Errorf("wrong diagnostic detail %#v; want %#v", got, want)
+				t.Errorf("wrong diagnostic detail\ngot: %s\nwant substring: %s", got, want)
 			}
 			if fromExpr := diags[0].FromExpr(); fromExpr != nil {
 				if fromExpr.Expression == nil {