diff --git a/aider/coders/base_coder.py b/aider/coders/base_coder.py
index 39f6357b4..a6b8cf0ef 100755
--- a/aider/coders/base_coder.py
+++ b/aider/coders/base_coder.py
@@ -1315,7 +1315,7 @@ class Coder:
             if len(rel_fnames) == 1 and fname in words:
                 mentioned_rel_fnames.add(rel_fnames[0])
 
-        return mentioned_rel_fnames
+        return set(mentioned_rel_fnames)
 
     def check_for_file_mentions(self, content):
         mentioned_rel_fnames = self.get_file_mentions(content)