diff -up mutt-1.5.21/sendlib.c.writehead mutt-1.5.21/sendlib.c
--- mutt-1.5.21/sendlib.c.writehead 2012-06-25 14:41:34.681483226 +0200
+++ mutt-1.5.21/sendlib.c 2012-06-25 14:41:44.485408610 +0200
@@ -1799,7 +1799,7 @@ static int write_one_header (FILE *fp, i
else
{
t = strchr (start, ':');
- if (t > end)
+ if (t == NULL || t > end)
{
dprint (1, (debugfile, "mwoh: warning: header not in "
"'key: value' format!\n"));