diff --git a/src/basic/socket-util.c b/src/basic/socket-util.c
index 2c70cade14..bd3cf6f869 100644
--- a/src/basic/socket-util.c
+++ b/src/basic/socket-util.c
@@ -1008,7 +1008,7 @@ int getpeergroups(int fd, gid_t **ret) {
         socklen_t n = sizeof(gid_t) * 64;
         _cleanup_free_ gid_t *d = NULL;
 
-        assert(fd);
+        assert(fd >= 0);
         assert(ret);
 
         for (;;) {