--- a/mercurial/formatter.py	Sun Oct 27 12:36:52 2019 +0900
+++ b/mercurial/formatter.py	Sun Oct 27 12:49:09 2019 +0900
@@ -515,6 +515,10 @@
         if part not in self._parts:
             return
         ref = self._parts[part]
+        # None can't be put in the mapping dict since it means <unset>
+        for k, v in item.items():
+            if v is None:
+                item[k] = templateutil.wrappedvalue(v)
         self._out.write(self._t.render(ref, item))
 
     @util.propertycache