Fix a typo.

PiperOrigin-RevId: 286320322
Change-Id: I69bc815219e3ee66792ed5ab991461673bc68c06
diff --git a/tensorflow/python/eager/context.py b/tensorflow/python/eager/context.py
index 540ee87..2a3fedc 100644
--- a/tensorflow/python/eager/context.py
+++ b/tensorflow/python/eager/context.py
@@ -1458,7 +1458,7 @@
     if policy is None:
       policy = MIRRORING_NONE
 
-    if self._mirroring_policy is not None or self._mirroring_policy != policy:
+    if self._mirroring_policy is None or self._mirroring_policy != policy:
       self._mirroring_policy = policy
 
       # Only set the policy if the context has already been initialized