diff --git a/index.js b/index.js
index 0ccfb82..f045b5c 100644
--- a/index.js
+++ b/index.js
@@ -72,7 +72,7 @@ class Parameter {
       var value = obj[key];
       var has = value !== null && value !== undefined;
 
-      if (!has) {
+      if (obj[key] === null || !has) {
         if (rule.required !== false) {
           errors.push({
             message: this.t('required'),