diff --git a/aider/commands.py b/aider/commands.py
index c3745a406..e177e77ca 100644
--- a/aider/commands.py
+++ b/aider/commands.py
@@ -711,11 +711,16 @@ class Commands:
                 self.io.tool_error(f"{matched_file} is already in the chat as an editable file")
                 continue
             elif abs_file_path in self.coder.abs_read_only_fnames:
-                self.coder.abs_read_only_fnames.remove(abs_file_path)
-                self.coder.abs_fnames.add(abs_file_path)
-                self.io.tool_output(
-                    f"Moved {matched_file} from read-only to editable files in the chat"
-                )
+                if self.coder.repo and self.coder.repo.path_in_repo(matched_file):
+                    self.coder.abs_read_only_fnames.remove(abs_file_path)
+                    self.coder.abs_fnames.add(abs_file_path)
+                    self.io.tool_output(
+                        f"Moved {matched_file} from read-only to editable files in the chat"
+                    )
+                else:
+                    self.io.tool_error(
+                        f"Cannot add {matched_file} as it's not part of the repository"
+                    )
             else:
                 if is_image_file(matched_file) and not self.coder.main_model.accepts_images:
                     self.io.tool_error(