diff --git a/src/pyj/ajax.pyj b/src/pyj/ajax.pyj
index 9337c6ad62..ec5c78cd1c 100644
--- a/src/pyj/ajax.pyj
+++ b/src/pyj/ajax.pyj
@@ -48,7 +48,7 @@ def absolute_path(path):
 
 
 def workaround_qt_bug(xhr, end_type, ok_code=200):
-    if end_type is 'error' and xhr.status is ok_code:
+    if end_type is 'error' and xhr.status is ok_code and window.navigator.userAgent.indexOf('calibre-viewer') is 0:
         end_type = 'load'
     return end_type