diff --git a/system/include/emscripten/val.h b/system/include/emscripten/val.h
index 197d8e8866302..dfb3e7e7c2b53 100644
--- a/system/include/emscripten/val.h
+++ b/system/include/emscripten/val.h
@@ -388,7 +388,7 @@ class EMBIND_VISIBILITY_DEFAULT val {
   }
 
   EM_VAL as_handle() const {
-#ifdef _REENTRANT
+#ifdef __EMSCRIPTEN_PTHREADS__
     assert(pthread_equal(thread, pthread_self()) && "val accessed from wrong thread");
 #endif
     return handle;
@@ -568,7 +568,7 @@ class EMBIND_VISIBILITY_DEFAULT val {
 private:
   // takes ownership, assumes handle already incref'd and lives on the same thread
   explicit val(EM_VAL handle) :
-#ifdef _REENTRANT
+#ifdef __EMSCRIPTEN_PTHREADS__
     thread(pthread_self()),
 #endif
     handle(handle) {}