diff --git a/source/sign.c b/source/sign.c
index 84586a2..c042d23 100644
--- a/source/sign.c
+++ b/source/sign.c
@@ -40,7 +40,7 @@ static int parse_args(struct args *args, int argc, char *argv[])
 	};
 	int ret;
 
-	while ((ret = getopt_long(argc, (char * const *)argv, "hi:d:k:", options,
+	while ((ret = getopt_long(argc, (char * const *)argv, "hi:d:k:c:", options,
 		NULL)) >= 0) {
 		switch (ret) {
 		case OPTION_HELP: return -1;