diff --git a/mercurial/wireprotov2server.py b/mercurial/wireprotov2server.py
--- a/mercurial/wireprotov2server.py
+++ b/mercurial/wireprotov2server.py
@@ -362,7 +362,7 @@
                 ', '.join(sorted(extra)))
 
         # And look for required arguments that are missing.
-        missing = {a for a in args if args[a].get('required')} - set(self._args)
+        missing = {a for a in args if args[a]['required']} - set(self._args)
 
         if missing:
             raise error.WireprotoCommandError(
@@ -374,7 +374,7 @@
         for k, meta in sorted(args.items()):
             # This argument wasn't passed by the client.
             if k not in self._args:
-                data[k] = meta.get('default', lambda: None)()
+                data[k] = meta['default']()
                 continue
 
             v = self._args[k]
@@ -543,7 +543,8 @@
           or ``bool``.
 
        ``default``
-          A callable returning the default value for this argument.
+          A callable returning the default value for this argument. If not
+          specified, ``None`` will be the default value.
 
        ``required``
           Bool indicating whether the argument is required.
@@ -600,6 +601,9 @@
             raise error.ProgrammingError('%s argument for command %s does not '
                                          'declare example field' % (arg, name))
 
+        meta.setdefault('default', lambda: None)
+        meta.setdefault('required', False)
+
     def register(func):
         if name in COMMANDS:
             raise error.ProgrammingError('%s command already registered '