diff --git a/src/JsonSchema/Constraints/FormatConstraint.php b/src/JsonSchema/Constraints/FormatConstraint.php
index 39c3eb8f..482ba668 100644
--- a/src/JsonSchema/Constraints/FormatConstraint.php
+++ b/src/JsonSchema/Constraints/FormatConstraint.php
@@ -176,7 +176,7 @@ public function check(&$element, $schema = null, ?JsonPointer $path = null, $i =
 
     protected function validateDateTime($datetime, $format)
     {
-        $dt = \DateTime::createFromFormat($format, $datetime);
+        $dt = \DateTime::createFromFormat($format, (string) $datetime);
 
         if (!$dt) {
             return false;
diff --git a/tests/Constraints/FormatTest.php b/tests/Constraints/FormatTest.php
index 98c0299e..2d91e5fe 100644
--- a/tests/Constraints/FormatTest.php
+++ b/tests/Constraints/FormatTest.php
@@ -186,7 +186,7 @@ public function getInvalidFormats(): array
             ['1999-01-11T00:00:00+1:00', 'date-time'],
             ['1999.000Z-01-11T00:00:00+1:00', 'date-time'],
 
-            [(string) PHP_INT_MAX, 'utc-millisec'],
+            [PHP_INT_MAX, 'utc-millisec'],
 
             ['grey', 'color'],
             ['#HHH', 'color'],