diff --git a/Lib/mailbox.py b/Lib/mailbox.py
index 520afeb..3f7a12a 100755
--- a/Lib/mailbox.py
+++ b/Lib/mailbox.py
@@ -459,7 +459,11 @@
         """Update table of contents mapping."""
         self._toc = {}
         for subdir in ('new', 'cur'):
-            for entry in os.listdir(os.path.join(self._path, subdir)):
+            subdir_path = os.path.join(self._path, subdir)
+            for entry in os.listdir(subdir_path):
+                p = os.path.join(subdir_path, entry)
+                if os.path.isdir(p):
+                    continue
                 uniq = entry.split(self.colon)[0]
                 self._toc[uniq] = os.path.join(subdir, entry)