diff --git a/lib/src/validator.dart b/lib/src/validator.dart
index 22f907160c11302f91002110f1f07d8a7d6d6694..672e3f62644f307bb7717b874829c65076d998e9 100644
--- a/lib/src/validator.dart
+++ b/lib/src/validator.dart
@@ -77,8 +77,9 @@ abstract class Validator {
 
     return Future.wait(validators.map((validator) => validator.validate()))
       .then((_) {
-      var errors = validators.expand((validator) => validator.errors);
-      var warnings = validators.expand((validator) => validator.warnings);
+      var errors = validators.expand((validator) => validator.errors).toList();
+      var warnings = validators.expand((validator) => validator.warnings)
+          .toList();
 
       if (!errors.isEmpty) {
         log.error("Missing requirements:");