diff --git a/src/Framework/Constraint/Count.php b/src/Framework/Constraint/Count.php
index 1ce9d3d0c23..194844c097c 100644
--- a/src/Framework/Constraint/Count.php
+++ b/src/Framework/Constraint/Count.php
@@ -106,7 +106,7 @@ protected function getCountOf($other)
             // moves pointer
             if ($key !== null) {
                 $iterator->rewind();
-                while ($key !== $iterator->key()) {
+                while ($iterator->valid() && $key !== $iterator->key()) {
                     $iterator->next();
                 }
             }