diff --git a/ext/ripper/lib/ripper/lexer.rb b/ext/ripper/lib/ripper/lexer.rb
index ffde9a4ed3e89a..ece71dd466442a 100644
--- a/ext/ripper/lib/ripper/lexer.rb
+++ b/ext/ripper/lib/ripper/lexer.rb
@@ -186,7 +186,7 @@ def compile(pattern)
       if m = /[^\w\s$()\[\]{}?*+\.]/.match(pattern)
         raise CompileError, "invalid char in pattern: #{m[0].inspect}"
       end
-      buf = ''
+      buf = +''
       pattern.scan(/(?:\w+|\$\(|[()\[\]\{\}?*+\.]+)/) do |tok|
         case tok
         when /\w/