diff --git a/nbase/snprintf.c b/nbase/snprintf.c
index a948d4f0a..f1c10a316 100644
--- a/nbase/snprintf.c
+++ b/nbase/snprintf.c
@@ -242,6 +242,8 @@ append_string (struct state *state,
                int prec,
                int flags)
 {
+  if(!arg)
+    arg = (unsigned char *) "(nall)";
   if(prec != -1)
     width -= prec;
   else