diff --git a/src/core/exec-invoke.c b/src/core/exec-invoke.c
index ff0fdbec5f..8b16acb399 100644
--- a/src/core/exec-invoke.c
+++ b/src/core/exec-invoke.c
@@ -3519,7 +3519,7 @@ static int close_remaining_fds(
                 const int *fds, size_t n_fds) {
 
         size_t n_dont_close = 0;
-        int dont_close[n_fds + 14];
+        int dont_close[n_fds + 15];
 
         assert(params);
 
@@ -3555,6 +3555,8 @@ static int close_remaining_fds(
         if (params->user_lookup_fd >= 0)
                 dont_close[n_dont_close++] = params->user_lookup_fd;
 
+        assert(n_dont_close <= ELEMENTSOF(dont_close));
+
         return close_all_fds(dont_close, n_dont_close);
 }