diff --git a/src/Runtime/LambdaRuntime.php b/src/Runtime/LambdaRuntime.php
index 4ce1e13bb..e97400e07 100755
--- a/src/Runtime/LambdaRuntime.php
+++ b/src/Runtime/LambdaRuntime.php
@@ -251,7 +251,7 @@ private function postJson(string $url, $data): void
          * - else we encode all other responses to base64
          * API Gateway checks `isBase64Encoded` and decodes what we returned if necessary.
          */
-        if ($contentType && $contentType !== 'application/json' && substr($contentType, 0, 5) !== 'text/') {
+        if ($contentType && $contentType !== 'application/json' && strpos($contentType, 'text/') !== 0) {
             $data['body'] = base64_encode($data['body']);
             $data['isBase64Encoded'] = true;
         }