diff --git a/tool/expand-config.rb b/tool/expand-config.rb
index 0b6fa54b30..bb88865709 100755
--- a/tool/expand-config.rb
+++ b/tool/expand-config.rb
@@ -9,7 +9,7 @@ config["RUBY_RELEASE_DATE"] ||=
 
 while /\A(\w+)=(.*)/ =~ ARGV[0]
   config[$1] = $2
-  config[$1].tr(File::ALT_SEPARATOR, File::SEPARATOR) if File::ALT_SEPARATOR
+  config[$1].tr!(File::ALT_SEPARATOR, File::SEPARATOR) if File::ALT_SEPARATOR
   ARGV.shift
 end