diff --git a/src/message.c b/src/message.c
index 4040e5b..7a6e346 100644
--- a/src/message.c
+++ b/src/message.c
@@ -307,7 +307,8 @@
     }
     else
     {
-	for (i = (int)STRLEN(s); len + (n = ptr2cells(s + i - 1)) <= room; --i)
+	for (i = (int)STRLEN(s);
+		   i - 1 >= 0 && len + (n = ptr2cells(s + i - 1)) <= room; --i)
 	    len += n;
     }