diff --git a/usr.sbin/ctld/ctld.cc b/usr.sbin/ctld/ctld.cc
--- a/usr.sbin/ctld/ctld.cc
+++ b/usr.sbin/ctld/ctld.cc
@@ -349,6 +349,8 @@
 		dm = 32;
 	}
 	if (mask != NULL) {
+		if (mask[0] == '\0')
+			goto error;
 		m = strtol(mask, &tmp, 0);
 		if (m < 0 || m > dm || tmp[0] != 0)
 			goto error;