diff --git a/Documentation/Makefile b/Documentation/Makefile
index d4c05ca879..1c9dfcea5a 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -108,7 +108,14 @@ endif
 # use MAN_BASE_URL=http://www.kernel.org/pub/software/scm/git/docs/
 # but distros may want to set it to /usr/share/doc/git-core/docs/ or
 # something like that.
+#
+# As older stylesheets simply ignore this parameter, it ought to be
+# safe to set it to empty string when the base URL is not specified,
+# but unfortunately we cannot do so unconditionally because at least
+# xmlto 0.0.18 is reported to lack --stringparam option.
+ifdef MAN_BASE_URL
 XMLTO_EXTRA += --stringparam man.base.url.for.relative.links=$(MAN_BASE_URL)
+endif
 
 # If your target system uses GNU groff, it may try to render
 # apostrophes as a "pretty" apostrophe using unicode.  This breaks