diff --git a/mercurial/store.py b/mercurial/store.py
index ba9df4e6abb8579f4c8590ceaa7609453392abf7_bWVyY3VyaWFsL3N0b3JlLnB5..9fbdf355c9167f594c3ed7de991d2529dd948423_bWVyY3VyaWFsL3N0b3JlLnB5 100644
--- a/mercurial/store.py
+++ b/mercurial/store.py
@@ -1120,7 +1120,7 @@
 class _fncachevfs(vfsmod.proxyvfs):
     def __init__(self, vfs, fnc, encode):
         vfsmod.proxyvfs.__init__(self, vfs)
-        self.fncache = fnc
+        self.fncache: fncache = fnc
         self.encode = encode
 
     def __call__(self, path, mode=b'r', *args, **kw):