diff --git a/examples/point.c b/examples/point.c
index 0386230..557be11 100644
--- a/examples/point.c
+++ b/examples/point.c
@@ -123,10 +123,11 @@ static const JSCFunctionListEntry js_point_proto_funcs[] = {
 static int js_point_init(JSContext *ctx, JSModuleDef *m)
 {
     JSValue point_proto, point_class;
+    JSRuntime *rt = JS_GetRuntime(ctx);
 
     /* create the Point class */
-    JS_NewClassID(&js_point_class_id);
-    JS_NewClass(JS_GetRuntime(ctx), js_point_class_id, &js_point_class);
+    JS_NewClassID(rt, &js_point_class_id);
+    JS_NewClass(rt, js_point_class_id, &js_point_class);
 
     point_proto = JS_NewObject(ctx);
     JS_SetPropertyFunctionList(ctx, point_proto, js_point_proto_funcs, countof(js_point_proto_funcs));
diff --git a/quickjs-libc.c b/quickjs-libc.c
index 4df3704..ea76222 100644
--- a/quickjs-libc.c
+++ b/quickjs-libc.c
@@ -1516,12 +1516,13 @@ static const JSCFunctionListEntry js_std_file_proto_funcs[] = {
 static int js_std_init(JSContext *ctx, JSModuleDef *m)
 {
     JSValue proto;
+    JSRuntime *rt = JS_GetRuntime(ctx);
 
     /* FILE class */
     /* the class ID is created once */
-    JS_NewClassID(&js_std_file_class_id);
+    JS_NewClassID(rt, &js_std_file_class_id);
     /* the class is created once per runtime */
-    JS_NewClass(JS_GetRuntime(ctx), js_std_file_class_id, &js_std_file_class);
+    JS_NewClass(rt, js_std_file_class_id, &js_std_file_class);
     proto = JS_NewObject(ctx);
     JS_SetPropertyFunctionList(ctx, proto, js_std_file_proto_funcs,
                                countof(js_std_file_proto_funcs));
@@ -3658,20 +3659,20 @@ static const JSCFunctionListEntry js_os_funcs[] = {
 
 static int js_os_init(JSContext *ctx, JSModuleDef *m)
 {
+    JSRuntime *rt = JS_GetRuntime(ctx);
     os_poll_func = js_os_poll;
 
     /* OSTimer class */
-    JS_NewClassID(&js_os_timer_class_id);
-    JS_NewClass(JS_GetRuntime(ctx), js_os_timer_class_id, &js_os_timer_class);
+    JS_NewClassID(rt, &js_os_timer_class_id);
+    JS_NewClass(rt, js_os_timer_class_id, &js_os_timer_class);
 
 #ifdef USE_WORKER
     {
-        JSRuntime *rt = JS_GetRuntime(ctx);
         JSThreadState *ts = JS_GetRuntimeOpaque(rt);
         JSValue proto, obj;
         /* Worker class */
-        JS_NewClassID(&js_worker_class_id);
-        JS_NewClass(JS_GetRuntime(ctx), js_worker_class_id, &js_worker_class);
+        JS_NewClassID(rt, &js_worker_class_id);
+        JS_NewClass(rt, js_worker_class_id, &js_worker_class);
         proto = JS_NewObject(ctx);
         JS_SetPropertyFunctionList(ctx, proto, js_worker_proto_funcs, countof(js_worker_proto_funcs));
 
diff --git a/quickjs.c b/quickjs.c
index 910708e..c2a8d12 100644
--- a/quickjs.c
+++ b/quickjs.c
@@ -228,6 +228,7 @@ struct JSRuntime {
     JSAtomStruct **atom_array;
     int atom_free_index; /* 0 = none */
 
+    JSClassID js_class_id_alloc; /* counter for user defined classes */
     int class_count;    /* size of class_array */
     JSClass *class_array;
 
@@ -1149,7 +1150,6 @@ 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 int compare_u32(uint32_t a, uint32_t b)
 {
@@ -1483,6 +1483,8 @@ JSRuntime *JS_NewRuntime2(const JSMallocFunctions *mf, void *opaque)
     if (init_shape_hash(rt))
         goto fail;
 
+    rt->js_class_id_alloc = JS_CLASS_INIT_COUNT;
+
     rt->stack_size = JS_DEFAULT_STACK_SIZE;
     JS_UpdateStackTop(rt);
 
@@ -3181,13 +3183,11 @@ static inline BOOL JS_IsEmptyString(JSValueConst v)
 /* JSClass support */
 
 /* a new class ID is allocated if *pclass_id != 0 */
-JSClassID JS_NewClassID(JSClassID *pclass_id)
+JSClassID JS_NewClassID(JSRuntime *rt, JSClassID *pclass_id)
 {
-    JSClassID class_id;
-    /* XXX: make it thread safe */
-    class_id = *pclass_id;
+    JSClassID class_id = *pclass_id;
     if (class_id == 0) {
-        class_id = js_class_id_alloc++;
+        class_id = rt->js_class_id_alloc++;
         *pclass_id = class_id;
     }
     return class_id;
diff --git a/quickjs.h b/quickjs.h
index aad0f18..a3bf90d 100644
--- a/quickjs.h
+++ b/quickjs.h
@@ -484,7 +484,7 @@ typedef struct JSClassDef {
     JSClassExoticMethods *exotic;
 } JSClassDef;
 
-JSClassID JS_NewClassID(JSClassID *pclass_id);
+JSClassID JS_NewClassID(JSRuntime *rt, JSClassID *pclass_id);
 int JS_NewClass(JSRuntime *rt, JSClassID class_id, const JSClassDef *class_def);
 int JS_IsRegisteredClass(JSRuntime *rt, JSClassID class_id);