diff --git a/ChangeLog b/ChangeLog
index df090df40e..34e00b1c29 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,4 +1,6 @@
-Thu Aug 30 13:13:13 2007  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+Thu Aug 30 13:49:25 2007  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* lib/mkmf.rb: initialize $top_srcdir always.  [ruby-dev:31682]
 
 	* lib/mkmf.rb (try_const, have_const): check for a const is defined.
 	  [ruby-core:04422]
diff --git a/lib/mkmf.rb b/lib/mkmf.rb
index 5794c16ce3..cb5bd9fc27 100644
--- a/lib/mkmf.rb
+++ b/lib/mkmf.rb
@@ -124,6 +124,7 @@ extdir = File.expand_path("ext", topdir)
 $extmk = File.expand_path($0)[0, extdir.size+1] == extdir+"/"
 if not $extmk and File.exist?(($hdrdir = RbConfig::CONFIG["rubyhdrdir"]) + "/ruby/ruby.h")
   $topdir = $hdrdir
+  $top_srcdir = $hdrdir
   $arch_hdrdir = $hdrdir + "/$(arch)"
 elsif File.exist?(($hdrdir = ($top_srcdir ||= topdir) + "/include")  + "/ruby.h") and
     File.exist?("#{CONFIG["EXTOUT"]}/include/#{CONFIG["arch"]}/ruby/config.h")