diff --git a/aider/coders/base_coder.py b/aider/coders/base_coder.py
index 22c8d8400..3960515af 100755
--- a/aider/coders/base_coder.py
+++ b/aider/coders/base_coder.py
@@ -493,9 +493,10 @@ class Coder:
             if content is not None:
                 all_content += content + "\n"
 
+        lines = all_content.splitlines()
         good = False
         for fence_open, fence_close in self.fences:
-            if fence_open in all_content or fence_close in all_content:
+            if any(line.startswith(fence_open) or line.startswith(fence_close) for line in lines):
                 continue
             good = True
             break