diff --git a/ext/json/ext/generator/generator.c b/ext/json/ext/generator/generator.c
index d810927c..517fa562 100644
--- a/ext/json/ext/generator/generator.c
+++ b/ext/json/ext/generator/generator.c
@@ -1848,6 +1848,8 @@ static VALUE cState_allow_duplicate_key_p(VALUE self)
             return Qnil;
         case JSON_RAISE:
             return Qfalse;
+        default:
+            return Qnil;
     }
 }