diff -r 2cdb82e8fb44 -r 89d93dd1a222 mercurial/store.py
--- a/mercurial/store.py	Sat Jul 14 02:09:47 2018 +0200
+++ b/mercurial/store.py	Sat Jul 14 02:10:43 2018 +0200
@@ -449,6 +449,7 @@
 
     def write(self, tr):
         if self._dirty:
+            assert self.entries is not None
             tr.addbackup('fncache')
             fp = self.vfs('fncache', mode='wb', atomictemp=True)
             if self.entries: