diff --git a/tools/mapmerge2/dmm.py b/tools/mapmerge2/dmm.py
index 5702ed21edc..bc12a39b355 100644
--- a/tools/mapmerge2/dmm.py
+++ b/tools/mapmerge2/dmm.py
@@ -95,7 +95,7 @@ class DMM:
         # last-second handling of bogus keys to help prevent and fix broken maps
         self._ensure_free_keys(0)
         max_key = max_key_for(self.key_length)
-        bad_keys = {key: 0 for key in self.dictionary.keys() if key > max_key}
+        bad_keys = {key: 0 for key in self.dictionary.keys() if key >= max_key}
         if bad_keys:
             print(f"Warning: fixing {len(bad_keys)} overflowing keys")
             for k in bad_keys: