1 files changed, 3 insertions, 1 deletions
diff --git a/ext/json/generator/generator.c b/ext/json/generator/generator.c
index e8b1476e31..d98879c60d 100644
--- a/ext/json/generator/generator.c
+++ b/ext/json/generator/generator.c
@@ -810,7 +810,9 @@ static void generate_json_string(FBuffer *buffer, VALUE Vstate, JSON_Generator_S
 {
     fbuffer_append_char(buffer, '"');
 #ifdef HAVE_RUBY_ENCODING_H
-    obj = rb_str_encode(obj, CEncoding_UTF_8, 0, Qnil);
+    if (!rb_enc_str_asciicompat_p(obj)) {
+        obj = rb_str_encode(obj, CEncoding_UTF_8, 0, Qnil);
+    }
 #endif
     if (state->ascii_only) {
         convert_UTF8_to_JSON_ASCII(buffer, obj);