diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -631,6 +631,9 @@ coreconfigitem('web', 'ipv6',
 coreconfigitem('web', 'port',
     default=8000,
 )
+coreconfigitem('web', 'prefix',
+    default='',
+)
 coreconfigitem('worker', 'backgroundclose',
     default=dynamicdefault,
 )
diff --git a/mercurial/hgweb/hgwebdir_mod.py b/mercurial/hgweb/hgwebdir_mod.py
--- a/mercurial/hgweb/hgwebdir_mod.py
+++ b/mercurial/hgweb/hgwebdir_mod.py
@@ -177,7 +177,7 @@ class hgwebdir(object):
         if self.stripecount:
             self.stripecount = int(self.stripecount)
         self._baseurl = self.ui.config('web', 'baseurl')
-        prefix = self.ui.config('web', 'prefix', '')
+        prefix = self.ui.config('web', 'prefix')
         if prefix.startswith('/'):
             prefix = prefix[1:]
         if prefix.endswith('/'):
diff --git a/mercurial/hgweb/server.py b/mercurial/hgweb/server.py
--- a/mercurial/hgweb/server.py
+++ b/mercurial/hgweb/server.py
@@ -277,7 +277,7 @@ class MercurialHTTPServer(_mixin, httpse
 
         handler.preparehttpserver(self, ui)
 
-        prefix = ui.config('web', 'prefix', '')
+        prefix = ui.config('web', 'prefix')
         if prefix:
             prefix = '/' + prefix.strip('/')
         self.prefix = prefix