Exclude directories in is_hardlink_master

src/borg/archive.py

@@ -990,7 +990,7 @@ Utilization of max. archive size: {csize_max:.0%}
             return 'source' not in item or not hardlinkable(item.mode) or item.source in hardlink_masters
 
         def is_hardlink_master(item):
-            return item.get('hardlink_master', True) and 'source' not in item
+            return item.get('hardlink_master', True) and 'source' not in item and hardlinkable(item.mode)
 
         def update_hardlink_masters(item1, item2):
             if is_hardlink_master(item1) or is_hardlink_master(item2):