diff --git a/evthread.c b/evthread.c
index 9b263b79..26d2a564 100644
--- a/evthread.c
+++ b/evthread.c
@@ -402,8 +402,9 @@ evthread_setup_global_lock_(void *lock_, unsigned locktype, int enable_locks)
 	} else {
 		/* Case 4: Fill in a debug lock with a real lock */
 		struct debug_lock *lock = lock_ ? lock_ : debug_lock_alloc(locktype);
-		EVUTIL_ASSERT(enable_locks &&
-		              evthread_lock_debugging_enabled_);
+		if (!lock)
+			return NULL;
+		EVUTIL_ASSERT(enable_locks && evthread_lock_debugging_enabled_);
 		EVUTIL_ASSERT(lock->locktype == locktype);
 		if (!lock->lock) {
 			lock->lock = original_lock_fns_.alloc(