diff --git a/src/Constraint/FormatConstraint.php b/src/Constraint/FormatConstraint.php
index 35e5758..ec98de4 100644
--- a/src/Constraint/FormatConstraint.php
+++ b/src/Constraint/FormatConstraint.php
@@ -122,6 +122,10 @@ private function isDateTimeValid($date, $format)
 
         $errors = DateTime::getLastErrors();
 
+        if (is_bool($errors)) {
+            return !$errors;
+        }
+
         return $errors['warning_count'] === 0 && $errors['error_count'] === 0;
     }
 }