diff --git a/ext/json/ext/generator/generator.c b/ext/json/ext/generator/generator.c
index e4412361..30d575db 100644
--- a/ext/json/ext/generator/generator.c
+++ b/ext/json/ext/generator/generator.c
@@ -740,7 +740,7 @@ json_object_i(VALUE key, VALUE val, VALUE _arg)
     long delim2_len = FBUFFER_LEN(state->object_delim2);
     long depth = state->depth;
     int j;
-    VALUE key_to_s;
+    VALUE klass, key_to_s;
 
     if (arg->iter > 0) fbuffer_append(buffer, delim, delim_len);
     if (object_nl) {
@@ -752,7 +752,14 @@ json_object_i(VALUE key, VALUE val, VALUE _arg)
         }
     }
 
-    key_to_s = rb_funcall(key, i_to_s, 0);
+    klass = CLASS_OF(key);
+    if (klass == rb_cString) {
+        key_to_s = key;
+    } else if (klass == rb_cSymbol) {
+        key_to_s = rb_id2str(SYM2ID(key));
+    } else {
+        key_to_s = rb_funcall(key, i_to_s, 0);
+    }
     Check_Type(key_to_s, T_STRING);
     generate_json(buffer, Vstate, state, key_to_s);
     fbuffer_append(buffer, delim2, delim2_len);