diff --git a/gen-unifi-cert.sh b/gen-unifi-cert.sh
index 0bbe04e..565d8f8 100755
--- a/gen-unifi-cert.sh
+++ b/gen-unifi-cert.sh
@@ -49,7 +49,7 @@ done
 
 MAINDOMAIN=${domains[0]}
 
-if [[ ! -z ${MAINDOMAIN} ]]; then
+if [[ -z ${MAINDOMAIN} ]]; then
 	echo "Error: At least one -d argument is required"
 	exit 1
 fi