diff --git a/api/js/jsapi/egw_json.js b/api/js/jsapi/egw_json.js
index 0e4ec1cf11..cd9c7b6ed5 100644
--- a/api/js/jsapi/egw_json.js
+++ b/api/js/jsapi/egw_json.js
@@ -537,7 +537,7 @@ egw.extend('json', egw.MODULE_WND_LOCAL, function(_app, _wnd)
 					// check if we need a not yet included app.js object --> include it now and return a Promise
 					else if (i == 1 && parts[0] == 'app' && typeof app.classes[parts[1]] === 'undefined')
 					{
-						this.includeJS('/'+parts[1]+'/js/app.js', undefined, undefined, this.webserverUrl)
+						return this.includeJS('/'+parts[1]+'/js/app.js', undefined, undefined, this.webserverUrl)
 							.then(() => this.applyFunc(_func, args, _context));
 					}
 					// check if we need a not yet instantiated app.js object --> instantiate it now