diff --git a/libc/src/__support/threads/thread.cpp b/libc/src/__support/threads/thread.cpp
index dad4f75f092ede..6f6b75be5766d0 100644
--- a/libc/src/__support/threads/thread.cpp
+++ b/libc/src/__support/threads/thread.cpp
@@ -117,7 +117,9 @@ class ThreadAtExitCallbackMgr {
 
   int add_callback(AtExitCallback *callback, void *obj) {
     cpp::lock_guard lock(mtx);
-    return callback_list.push_back({callback, obj});
+    if (callback_list.push_back({callback, obj}))
+      return 0;
+    return -1;
   }
 
   void call() {