diff --git a/src/Runtime/LambdaRuntime.php b/src/Runtime/LambdaRuntime.php
index dbf1f68dd..a42e28b5b 100755
--- a/src/Runtime/LambdaRuntime.php
+++ b/src/Runtime/LambdaRuntime.php
@@ -244,7 +244,7 @@ public function failInitialization(string $message, ?\Throwable $error = null):
     private function postJson(string $url, $data): void
     {
         $contentType = $data['multiValueHeaders']['content-type'][0] ?? null;
-        if ($contentType && $contentType !== 'application/json' && substr($contentType, 0, 4) !== 'text') {
+        if ($contentType && $contentType !== 'application/json' && substr($contentType, 0, 5) !== 'text/') {
             $data['body'] = base64_encode($data['body']);
             $data['isBase64Encoded'] = true;
         }