diff --git a/json.php b/json.php
index 28736362dd..3ae33cc8a2 100644
--- a/json.php
+++ b/json.php
@@ -100,6 +100,7 @@ if (isset($_GET['menuaction']))
 	//Check whether the request data is set
 	if (isset($GLOBALS['egw_unset_vars']['_POST[json_data]']))
 	{
+		$json->isJSONRequest(true);	// otherwise exception is not send back to client, as we have not yet called parseRequest()
 		throw new egw_exception_assertion_failed("JSON Data contains script tags. Aborting...");
 	}
 	$json->parseRequest($_GET['menuaction'], $_REQUEST['json_data']);