diff --git a/doc/gendoc.py b/doc/gendoc.py
--- a/doc/gendoc.py
+++ b/doc/gendoc.py
@@ -31,6 +31,7 @@
     commands,
     encoding,
     extensions,
+    fancyopts,
     help,
     minirst,
     pycompat,
@@ -86,6 +87,8 @@
         if b'\n' in desc:
             # only remove line breaks and indentation
             desc = b' '.join(l.lstrip() for l in desc.split(b'\n'))
+        if isinstance(default, fancyopts.customopt):
+            default = default.getdefaultvalue()
         if default:
             default = stringutil.forcebytestr(default)
             desc += _(b" (default: %s)") % default