diff --git a/src/basic/fd-util.c b/src/basic/fd-util.c
index b623dc7ff1..c2d46aae15 100644
--- a/src/basic/fd-util.c
+++ b/src/basic/fd-util.c
@@ -167,6 +167,8 @@ int fd_nonblock(int fd, bool nonblock) {
 }
 
 void nonblock_resetp(int *fd) {
+        assert(fd);
+
         PROTECT_ERRNO;
 
         if (*fd >= 0)