diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -1876,7 +1876,7 @@
     for section, name, value in walkconfig:
         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')
@@ -1885,8 +1885,8 @@
             continue
         fm.startitem()
         fm.condwrite(ui.debugflag, 'source', '%s: ', source)
+        fm.data(name=entryname, defaultvalue=defaultvalue)
         if uniquesel:
-            fm.data(name=entryname, defaultvalue=defaultvalue)
             fm.write('value', '%s\n', value)
         else:
             fm.write('name value', '%s=%s\n', entryname, value)
diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -792,7 +792,10 @@
                 itemdefault = item.default()
             else:
                 itemdefault = item.default
-        return itemdefault
+            if item.default is configitems.dynamicdefault:
+                self.warn(_("config item requires an explicit default value: "
+                            "'%s.%s'\n" % (section, name)))
+            return itemdefault
 
     def hasconfig(self, section, name, untrusted=False):
         return self._data(untrusted).hasitem(section, name)
diff --git a/tests/test-config.t b/tests/test-config.t
--- a/tests/test-config.t
+++ b/tests/test-config.t
@@ -57,11 +57,13 @@
   $ hg showconfig Section -Tjson
   [
    {
+    "defaultvalue": null,
     "name": "Section.KeY",
     "source": "*.hgrc:*", (glob)
     "value": "Case Sensitive"
    },
    {
+    "defaultvalue": null,
     "name": "Section.key",
     "source": "*.hgrc:*", (glob)
     "value": "lower case"
@@ -70,15 +72,15 @@
   $ hg showconfig Section.KeY -Tjson
   [
    {
-    "defaultvalue": "None",
+    "defaultvalue": null,
     "name": "Section.KeY",
     "source": "*.hgrc:*", (glob)
     "value": "Case Sensitive"
    }
   ]
   $ hg showconfig -Tjson | tail -7
-   },
    {
+    "defaultvalue": null,
     "name": "*", (glob)
     "source": "*", (glob)
     "value": "*" (glob)
@@ -103,7 +105,7 @@
   $ hg config empty.source -Tjson
   [
    {
-    "defaultvalue": "None",
+    "defaultvalue": null,
     "name": "empty.source",
     "source": "",
     "value": "value"
@@ -170,15 +172,19 @@
 config affected by environment variables
 
   $ EDITOR=e1 VISUAL=e2 hg config --debug | grep 'ui\.editor'
+  config item requires an explicit default value: 'ui.editor'
   $VISUAL: ui.editor=e2
 
   $ VISUAL=e2 hg config --debug --config ui.editor=e3 | grep 'ui\.editor'
+  config item requires an explicit default value: 'ui.editor'
   --config: ui.editor=e3
 
   $ PAGER=p1 hg config --debug | grep 'pager\.pager'
+  config item requires an explicit default value: 'pager.pager'
   $PAGER: pager.pager=p1
 
   $ PAGER=p1 hg config --debug --config pager.pager=p2 | grep 'pager\.pager'
+  config item requires an explicit default value: 'pager.pager'
   --config: pager.pager=p2
 
 verify that aliases are evaluated as well