diff --git a/aider/commands.py b/aider/commands.py
index 31d6f8044..29f4bc63b 100644
--- a/aider/commands.py
+++ b/aider/commands.py
@@ -279,7 +279,7 @@ class Commands:
 
         if not args.strip():
             self.io.tool_output("Dropping all files from the chat session.")
-            self.coder.abs_fnames = []
+            self.coder.abs_fnames = set()
 
         for word in args.split():
             matched_files = [