diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -1872,7 +1872,7 @@
     for section, name, value in ui.walkconfig(untrusted=untrusted):
         source = ui.configsource(section, name, untrusted)
         value = pycompat.bytestr(value)
-        defaultvalue = pycompat.bytestr(ui.configdefault(section, name))
+        defaultvalue = ui.configdefault(section, name)
         if fm.isplain():
             source = source or 'none'
             value = value.replace('\n', '\\n')
diff --git a/tests/test-config.t b/tests/test-config.t
--- a/tests/test-config.t
+++ b/tests/test-config.t
@@ -70,7 +70,7 @@
   $ hg showconfig Section.KeY -Tjson
   [
    {
-    "defaultvalue": "None",
+    "defaultvalue": null,
     "name": "Section.KeY",
     "source": "*.hgrc:*", (glob)
     "value": "Case Sensitive"
@@ -103,7 +103,7 @@
   $ hg config empty.source -Tjson
   [
    {
-    "defaultvalue": "None",
+    "defaultvalue": null,
     "name": "empty.source",
     "source": "",
     "value": "value"