diff --git a/src/posix.c b/src/posix.c
index fcd46161..adbdda42 100644
--- a/src/posix.c
+++ b/src/posix.c
@@ -251,8 +251,9 @@ open_output_file (SCM x)        /*:((arity . n)) */
   SCM file_name = car (x);
   x = cdr (x);
   int mode = S_IRUSR | S_IWUSR;
-  if (TYPE (x) == TPAIR && TYPE (car (x)) == TNUMBER)
-    mode = VALUE (car (x));
+  if (TYPE (x) == TPAIR)
+    if (TYPE (car (x)) == TNUMBER)
+      mode = VALUE (car (x));
   return make_number (mes_open (cell_bytes (STRING (file_name)), O_WRONLY | O_CREAT | O_TRUNC, mode));
 }