diff --git a/src/Mod/Part/AttachmentEditor/TaskAttachmentEditor.py b/src/Mod/Part/AttachmentEditor/TaskAttachmentEditor.py
index 5c2f4e8ab5..fbcb6f4fd8 100644
--- a/src/Mod/Part/AttachmentEditor/TaskAttachmentEditor.py
+++ b/src/Mod/Part/AttachmentEditor/TaskAttachmentEditor.py
@@ -66,7 +66,7 @@ def linkSubList_convertToOldStyle(references):
                 result.append((tup[0], subname))
             if len(tup[1]) == 0:
                 result.append((tup[0], ''))
-        elif isinstance(tup[1],basestring):
+        elif isinstance(tup[1],str):
             # old style references, no conversion required
             result.append(tup)
     return result