diff --git a/mercurial/formatter.py b/mercurial/formatter.py
--- a/mercurial/formatter.py
+++ b/mercurial/formatter.py
@@ -515,6 +515,10 @@ class templateformatter(baseformatter):
         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
diff --git a/tests/test-config.t b/tests/test-config.t
--- a/tests/test-config.t
+++ b/tests/test-config.t
@@ -122,7 +122,7 @@ Test config default of various types:
   ]
   $ hg config --config auth.cookiefile= auth -T'json(defaultvalue)'
   [
-   {"defaultvalue": ""}
+   {"defaultvalue": null}
   ]
   $ hg config --config auth.cookiefile= auth -T'{defaultvalue}\n'