diff --git a/cmd/protoc-gen-go/internal_gengo/well_known_types.go b/cmd/protoc-gen-go/internal_gengo/well_known_types.go
index 9a1b7bd..dbaa529 100644
--- a/cmd/protoc-gen-go/internal_gengo/well_known_types.go
+++ b/cmd/protoc-gen-go/internal_gengo/well_known_types.go
@@ -971,9 +971,12 @@
 		g.P()
 		g.P("			// Identify the next message to search within.")
 		g.P("			md = fd.Message() // may be nil")
-		g.P("			if fd.IsMap() {")
-		g.P("				md = fd.MapValue().Message() // may be nil")
+		g.P()
+		g.P("			// Repeated fields are only allowed at the last postion.")
+		g.P("			if fd.IsList() || fd.IsMap() {")
+		g.P("				md = nil")
 		g.P("			}")
+		g.P()
 		g.P("			return true")
 		g.P("		}) {")
 		g.P("			return i")
diff --git a/types/known/fieldmaskpb/field_mask.pb.go b/types/known/fieldmaskpb/field_mask.pb.go
index 6a8d872..a852bef 100644
--- a/types/known/fieldmaskpb/field_mask.pb.go
+++ b/types/known/fieldmaskpb/field_mask.pb.go
@@ -393,9 +393,12 @@
 
 			// Identify the next message to search within.
 			md = fd.Message() // may be nil
-			if fd.IsMap() {
-				md = fd.MapValue().Message() // may be nil
+
+			// Repeated fields are only allowed at the last postion.
+			if fd.IsList() || fd.IsMap() {
+				md = nil
 			}
+
 			return true
 		}) {
 			return i
diff --git a/types/known/fieldmaskpb/field_mask_test.go b/types/known/fieldmaskpb/field_mask_test.go
index 6d21711..19756c5 100644
--- a/types/known/fieldmaskpb/field_mask_test.go
+++ b/types/known/fieldmaskpb/field_mask_test.go
@@ -37,7 +37,8 @@
 	}, {
 		inMessage: (*testpb.TestAllTypes)(nil),
 		inPaths:   []string{"optional_int32", "OptionalGroup.optional_nested_message", "map_uint32_uint32", "map_string_nested_message.corecursive", "oneof_bool"},
-		wantPaths: []string{"optional_int32", "OptionalGroup.optional_nested_message", "map_uint32_uint32", "map_string_nested_message.corecursive", "oneof_bool"},
+		wantPaths: []string{"optional_int32", "OptionalGroup.optional_nested_message", "map_uint32_uint32"},
+		wantError: cmpopts.AnyError,
 	}, {
 		inMessage: (*testpb.TestAllTypes)(nil),
 		inPaths:   []string{"optional_nested_message", "optional_nested_message.corecursive", "optional_nested_message.corecursive.optional_nested_message", "optional_nested_message.corecursive.optional_nested_message.corecursive"},
@@ -194,3 +195,144 @@
 		})
 	}
 }
+
+func TestIsValid(t *testing.T) {
+	tests := []struct {
+		message proto.Message
+		paths   []string
+		want    bool
+	}{{
+		message: (*testpb.TestAllTypes)(nil),
+		paths:   []string{"no_such_field"},
+		want:    false,
+	}, {
+		message: (*testpb.TestAllTypes)(nil),
+		paths:   []string{""},
+		want:    false,
+	}, {
+		message: (*testpb.TestAllTypes)(nil),
+		paths: []string{
+			"optional_int32",
+			"optional_int32",
+			"optional_int64",
+			"optional_uint32",
+			"optional_uint64",
+			"optional_sint32",
+			"optional_sint64",
+			"optional_fixed32",
+			"optional_fixed64",
+			"optional_sfixed32",
+			"optional_sfixed64",
+			"optional_float",
+			"optional_double",
+			"optional_bool",
+			"optional_string",
+			"optional_bytes",
+			"OptionalGroup",
+			"optional_nested_message",
+			"optional_foreign_message",
+			"optional_import_message",
+			"optional_nested_enum",
+			"optional_foreign_enum",
+			"optional_import_enum",
+			"repeated_int32",
+			"repeated_int64",
+			"repeated_uint32",
+			"repeated_uint64",
+			"repeated_sint32",
+			"repeated_sint64",
+			"repeated_fixed32",
+			"repeated_fixed64",
+			"repeated_sfixed32",
+			"repeated_sfixed64",
+			"repeated_float",
+			"repeated_double",
+			"repeated_bool",
+			"repeated_string",
+			"repeated_bytes",
+			"RepeatedGroup",
+			"repeated_nested_message",
+			"repeated_foreign_message",
+			"repeated_importmessage",
+			"repeated_nested_enum",
+			"repeated_foreign_enum",
+			"repeated_importenum",
+			"map_int32_int32",
+			"map_int64_int64",
+			"map_uint32_uint32",
+			"map_uint64_uint64",
+			"map_sint32_sint32",
+			"map_sint64_sint64",
+			"map_fixed32_fixed32",
+			"map_fixed64_fixed64",
+			"map_sfixed32_sfixed32",
+			"map_sfixed64_sfixed64",
+			"map_int32_float",
+			"map_int32_double",
+			"map_bool_bool",
+			"map_string_string",
+			"map_string_bytes",
+			"map_string_nested_message",
+			"map_string_nested_enum",
+			"oneof_uint32",
+			"oneof_nested_message",
+			"oneof_string",
+			"oneof_bytes",
+			"oneof_bool",
+			"oneof_uint64",
+			"oneof_float",
+			"oneof_double",
+			"oneof_enum",
+			"OneofGroup",
+		},
+		want: true,
+	}, {
+		message: (*testpb.TestAllTypes)(nil),
+		paths: []string{
+			"optional_nested_message.a",
+			"optional_nested_message.corecursive",
+			"optional_nested_message.corecursive.optional_int32",
+			"optional_nested_message.corecursive.optional_nested_message.corecursive.optional_nested_message.a",
+			"OptionalGroup.a",
+			"OptionalGroup.optional_nested_message",
+			"OptionalGroup.optional_nested_message.corecursive",
+			"oneof_nested_message.a",
+			"oneof_nested_message.corecursive",
+		},
+		want: true,
+	}, {
+		message: (*testpb.TestAllTypes)(nil),
+		paths:   []string{"repeated_nested_message.a"},
+		want:    false,
+	}, {
+		message: (*testpb.TestAllTypes)(nil),
+		paths:   []string{"repeated_nested_message[0]"},
+		want:    false,
+	}, {
+		message: (*testpb.TestAllTypes)(nil),
+		paths:   []string{"repeated_nested_message[0].a"},
+		want:    false,
+	}, {
+		message: (*testpb.TestAllTypes)(nil),
+		paths:   []string{"map_string_nested_message.a"},
+		want:    false,
+	}, {
+		message: (*testpb.TestAllTypes)(nil),
+		paths:   []string{`map_string_nested_message["key"]`},
+		want:    false,
+	}, {
+		message: (*testpb.TestAllExtensions)(nil),
+		paths:   []string{"nested_string_extension"},
+		want:    false,
+	}}
+
+	for _, tt := range tests {
+		t.Run("", func(t *testing.T) {
+			mask := &fmpb.FieldMask{Paths: tt.paths}
+			got := mask.IsValid(tt.message)
+			if got != tt.want {
+				t.Errorf("IsValid() returns %v want %v", got, tt.want)
+			}
+		})
+	}
+}