diff --git a/aider/coders/base_coder.py b/aider/coders/base_coder.py
index 5622ce0a1..557cc83ad 100755
--- a/aider/coders/base_coder.py
+++ b/aider/coders/base_coder.py
@@ -307,7 +307,7 @@ class Coder:
             self.fence = self.fences[0]
             self.io.tool_error(
                 "Unable to find a fencing strategy! Falling back to:"
-                " {self.fence[0]}...{self.fence[1]}"
+                f" {self.fence[0]}...{self.fence[1]}"
             )
 
         return