diff --git a/quickjs.c b/quickjs.c
index 244cc0e..c7616cb 100644
--- a/quickjs.c
+++ b/quickjs.c
@@ -1314,7 +1314,7 @@ static const JSClassExoticMethods js_arguments_exotic_methods;
 static const JSClassExoticMethods js_string_exotic_methods;
 static const JSClassExoticMethods js_proxy_exotic_methods;
 static const JSClassExoticMethods js_module_ns_exotic_methods;
-static JSClassID js_class_id_alloc = JS_CLASS_INIT_COUNT;
+static _Thread_local JSClassID js_class_id_alloc = JS_CLASS_INIT_COUNT;
 
 static void js_trigger_gc(JSRuntime *rt, size_t size)
 {