diff --git a/zipl/boot/stage3.c b/zipl/boot/stage3.c
index 115a2085..250c50e8 100644
--- a/zipl/boot/stage3.c
+++ b/zipl/boot/stage3.c
@@ -199,7 +199,10 @@ void start(void)
 	 * if extra parm string starts with '=' replace original string,
 	 * else append
 	 */
-	if (*cextra == 0x3d) {
+	if (*cextra == 0x3d && cextra_len >= 1) {
+		/* skip '=' */
+		cextra++;
+		cextra_len--;
 		memcpy(cmdline, cextra, cextra_len);
 	} else if (cmdline_len + 1 <= COMMAND_LINE_SIZE - 1) {
 		/* add blank */