diff --git a/pandas/core/indexes/multi.py b/pandas/core/indexes/multi.py
index b4e8a763d1210..debdcdcc3f762 100644
--- a/pandas/core/indexes/multi.py
+++ b/pandas/core/indexes/multi.py
@@ -3534,7 +3534,7 @@ def _union(self, other, sort) -> MultiIndex:
         other, result_names = self._convert_can_do_setop(other)
         if (
             any(-1 in code for code in self.codes)
-            and any(-1 in code for code in self.codes)
+            and any(-1 in code for code in other.codes)
             or self.has_duplicates
             or other.has_duplicates
         ):