diff --git i/ext/json/generator/generator.c w/ext/json/generator/generator.c
index b1babb1..c282515 100644
--- i/ext/json/generator/generator.c
+++ w/ext/json/generator/generator.c
@@ -16,7 +16,7 @@ static ID i_to_s, i_to_json, i_new, i_indent, i_space, i_space_before,
           i_object_nl, i_array_nl, i_max_nesting, i_allow_nan, i_ascii_only,
           i_quirks_mode, i_pack, i_unpack, i_create_id, i_extend, i_key_p,
           i_aref, i_send, i_respond_to_p, i_match, i_keys, i_depth,
-          i_buffer_initial_length, i_dup;
+          i_buffer_initial_length, i_dup, i_valid_encoding_p;
 
 /*
  * Copyright 2001-2004 Unicode, Inc.
@@ -726,11 +726,13 @@ static void generate_json_array(FBuffer *buffer, VALUE Vstate, JSON_Generator_St
 
 static void generate_json_string(FBuffer *buffer, VALUE Vstate, JSON_Generator_State *state, VALUE obj)
 {
+    int ascii = state->ascii_only;
     fbuffer_append_char(buffer, '"');
 #ifdef HAVE_RUBY_ENCODING_H
     obj = rb_funcall(obj, i_encode, 1, CEncoding_UTF_8);
+    if (!ascii && !RTEST(rb_funcall(obj, i_valid_encoding_p, 0, 0))) ascii = 1;
 #endif
-    if (state->ascii_only) {
+    if (ascii) {
         convert_UTF8_to_JSON_ASCII(buffer, obj);
     } else {
         convert_UTF8_to_JSON(buffer, obj);
@@ -1387,6 +1389,7 @@ void Init_generator()
     CEncoding_UTF_8 = rb_funcall(rb_path2class("Encoding"), rb_intern("find"), 1, rb_str_new2("utf-8"));
     i_encoding = rb_intern("encoding");
     i_encode = rb_intern("encode");
+    i_valid_encoding_p = rb_intern("valid_encoding?");
 #endif
     i_SAFE_STATE_PROTOTYPE = rb_intern("SAFE_STATE_PROTOTYPE");
     CJSON_SAFE_STATE_PROTOTYPE = Qnil;