diff --git a/feature_stream_string.go b/feature_stream_string.go
index 8655f93..334282f 100644
--- a/feature_stream_string.go
+++ b/feature_stream_string.go
@@ -286,8 +286,11 @@ func writeStringSlowPathWithHTMLEscaped(stream *Stream, i int, s string, valLen
 		}
 		c, size := utf8.DecodeRuneInString(s[i:])
 		if c == utf8.RuneError && size == 1 {
+			if start < i {
+				stream.WriteRaw(s[start:i])
+			}
+			stream.WriteRaw(`\ufffd`)
 			i++
-			stream.WriteRaw(s[start:i])
 			start = i
 			continue
 		}
diff --git a/jsoniter_invalid_test.go b/jsoniter_invalid_test.go
index ba864b4..338f4db 100644
--- a/jsoniter_invalid_test.go
+++ b/jsoniter_invalid_test.go
@@ -122,8 +122,11 @@ func Test_invalid_number(t *testing.T) {
 	obj := Message{}
 	decoder := ConfigCompatibleWithStandardLibrary.NewDecoder(bytes.NewBufferString(`{"number":"5"}`))
 	err := decoder.Decode(&obj)
-	result, err := ConfigCompatibleWithStandardLibrary.Marshal(err.Error())
+	invalidStr := err.Error()
+	result, err := ConfigCompatibleWithStandardLibrary.Marshal(invalidStr)
 	should := require.New(t)
 	should.Nil(err)
-	should.Contains(string(result), "\xff")
+	result2, err := json.Marshal(invalidStr)
+	should.Nil(err)
+	should.Equal(string(result2), string(result))
 }