mercurial/commandserver.py

--- a/mercurial/commandserver.py	Sun Nov 04 12:38:49 2018 +0900
+++ b/mercurial/commandserver.py	Sun Nov 04 12:51:14 2018 +0900
@@ -93,7 +93,8 @@
 
     def write(self, data, **opts):
         opts = pycompat.byteskwargs(opts)
-        opts[b'data'] = data
+        if data is not None:
+            opts[b'data'] = data
         self._cout.write(self._encodefn(opts))
 
     def __getattr__(self, attr):