Index: b/src/sexp.c
===================================================================
--- b/src/sexp.c
+++ b/src/sexp.c
@@ -1068,7 +1068,7 @@
     }
   c.pos = c.sexp->d;
 
-  for (p = buffer, n = length; n; p++, n--)
+  for (p = buffer, n = length; p!=NULL &&  n; p++, n--)
     {
       if (tokenp && !hexfmt)
 	{