diff --git a/mercurial/util.py b/mercurial/util.py
--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -1480,6 +1480,8 @@
             if default is _notset:
                 raise
             return default
+
+        assert node is not None  # help pytype
         value = node.value
         self.totalcost -= node.cost
         node.markempty()
@@ -1567,6 +1569,8 @@
         while n.key is _notset:
             n = n.prev
 
+        assert n is not None  # help pytype
+
         key, value = n.key, n.value
 
         # And remove it from the cache and mark it as empty.