diff -Naurp a/lib/util/debug.c b/lib/util/debug.c
--- a/lib/util/debug.c	2017-01-11 08:55:14.000000000 +0100
+++ b/lib/util/debug.c	2017-04-15 18:39:21.939188030 +0200
@@ -397,7 +397,7 @@ static void debug_backends_log(const cha
 	 * a buffer without the newline character.
 	 */
 	len = MIN(strlen(msg), FORMAT_BUFR_SIZE - 1);
-	if (msg[len - 1] == '\n') {
+	if ((len > 0) && (msg[len - 1] == '\n')) {
 		len--;
 	}