diff --git a/src/Codeception/Lib/InnerBrowser.php b/src/Codeception/Lib/InnerBrowser.php
index 181f5f0..d039c61 100644
--- a/src/Codeception/Lib/InnerBrowser.php
+++ b/src/Codeception/Lib/InnerBrowser.php
@@ -97,7 +97,7 @@ public function _failed(TestInterface $test, $fail)
             $internalResponse = false;
         }
 
-        $responseContentType = $internalResponse ? $internalResponse->getHeader('content-type') : '';
+        $responseContentType = $internalResponse ? (string) $internalResponse->getHeader('content-type') : '';
         [$responseMimeType] = explode(';', $responseContentType);
 
         $extension = $extensions[$responseMimeType] ?? 'html';