diff --git a/lib/api/document.php b/lib/api/document.php
--- a/lib/api/document.php
+++ b/lib/api/document.php
@@ -262,7 +262,12 @@
 
             $result['owner']      = $session['owner'];
             $result['owner_name'] = $session['owner_name'];
-            $result['user']       = $rcube->user->get_username();
+            if ($rcube && $rcube->user) {
+                $result['user']       = $rcube->user->get_username();
+            } else {
+                //Fall back to the owner if we don't have the rcube user initialized (wopi with kolabfiles)
+                $result['user']       = $session['owner'];
+            }
             $result['readonly']   = !empty($session['readonly']);
             $result['origin']     = $session['origin'];