diff --git a/libc/test/integration/src/pthread/pthread_once_test.cpp b/libc/test/integration/src/pthread/pthread_once_test.cpp
--- a/libc/test/integration/src/pthread/pthread_once_test.cpp
+++ b/libc/test/integration/src/pthread/pthread_once_test.cpp
@@ -28,7 +28,7 @@
 
 static void *func(void *) {
   static pthread_once_t flag = PTHREAD_ONCE_INIT;
-  __llvm_libc::pthread_once(&flag, pthread_once_func);
+  ASSERT_EQ(__llvm_libc::pthread_once(&flag, pthread_once_func), 0);
 
   thread_count.fetch_add(1);