diff --git a/lib/isc/task.c b/lib/isc/task.c
index 7559a67b73..4d030ae243 100644
--- a/lib/isc/task.c
+++ b/lib/isc/task.c
@@ -722,6 +722,7 @@ isc_taskmgr_create(isc_mem_t *mctx, unsigned int workers,
 	isc_thread_t *threads;
 
 	REQUIRE(workers > 0);
+	REQUIRE(managerp != NULL && *managerp == NULL);
 
 	manager = isc_mem_get(mctx, sizeof *manager);
 	if (manager == NULL)