diff --git a/lib/sensitive.rb b/lib/sensitive.rb
index 7219426..16ae006 100644
--- a/lib/sensitive.rb
+++ b/lib/sensitive.rb
@@ -10,7 +10,7 @@ class Sensitive
   end
 
   def mask(v)
-    "".concat(v[0, @head], @ch * (v.length - (@head + @tail)), v[-@tail, @tail])
+    +"".concat(v[0, @head], @ch * (v.length - (@head + @tail)), v[-@tail, @tail])
   end
 
   def unwrap