diff --git a/ChangeLog b/ChangeLog
index 7a4ba76a47..063febbb5d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Thu Nov 15 17:07:54 2007  Tanaka Akira  <akr@fsij.org>
+
+	* tool/compile_prelude.rb: fix first substitution.
+	  use constant for prefix.
+
 Thu Nov 15 14:29:56 2007  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* common.mk (prereq): added auto generated sources.  [ruby-dev:32280]
diff --git a/tool/compile_prelude.rb b/tool/compile_prelude.rb
index ea77729311..4d08b4d68a 100644
--- a/tool/compile_prelude.rb
+++ b/tool/compile_prelude.rb
@@ -29,6 +29,7 @@ lines_list = preludes.map {|filename|
   need_ruby_prefix = false
   File.readlines(filename).each {|line|
     line.gsub!(/RbConfig::CONFIG\["(\w+)"\]/) {
+      key = $1
       unless mkconf
         require 'rbconfig'
         mkconf = RbConfig::MAKEFILE_CONFIG.merge('prefix'=>'#{TMP_RUBY_PREFIX}')
@@ -36,12 +37,12 @@ lines_list = preludes.map {|filename|
         setup_ruby_prefix = "TMP_RUBY_PREFIX = $:.grep(%r{\\A/}).last[0..#{-exlen-1}]\n"
         teardown_ruby_prefix = 'Object.class_eval { remove_const "TMP_RUBY_PREFIX" }'
       end
-      if RbConfig::MAKEFILE_CONFIG.has_key? $1
-        val = RbConfig.expand("$(#$1)", mkconf)
+      if RbConfig::MAKEFILE_CONFIG.has_key? key
+        val = RbConfig.expand("$(#{key})", mkconf)
         need_ruby_prefix = true if /\A\#{TMP_RUBY_PREFIX\}/ =~ val
         c_esc(val)
       else
-        $&
+        "nil"
       end
     }
     lines << c_esc(line)