diff -r b7a75b9a3386 -r 0407a51b9d8c mercurial/commandserver.py
--- a/mercurial/commandserver.py	Tue Jul 11 08:52:55 2017 -0700
+++ b/mercurial/commandserver.py	Fri Jul 14 14:22:40 2017 -0700
@@ -156,7 +156,7 @@
         self.cwd = pycompat.getcwd()
 
         # developer config: cmdserver.log
-        logpath = ui.config("cmdserver", "log", None)
+        logpath = ui.config("cmdserver", "log")
         if logpath:
             global logfile
             if logpath == '-':