diff --git a/packages/common/pipes/validation.pipe.ts b/packages/common/pipes/validation.pipe.ts
index 11de63be218..ede40fb80eb 100644
--- a/packages/common/pipes/validation.pipe.ts
+++ b/packages/common/pipes/validation.pipe.ts
@@ -207,6 +207,9 @@ export class ValidationPipe implements PipeTransform<any> {
       return +value;
     }
     if (metatype === String) {
+      if (isUndefined(value)) {
+        return undefined;
+      }
       return String(value);
     }
     return value;