diff --git a/lib/yell/formatter.rb b/lib/yell/formatter.rb
index 3c9c2dc..76e1992 100644
--- a/lib/yell/formatter.rb
+++ b/lib/yell/formatter.rb
@@ -201,7 +201,7 @@ def call(event, time = nil, progname = nil, msg = nil)
     end
 
     def to_sprintf( table )
-      buff, args, _pattern = "", [], @pattern.dup
+      buff, args, _pattern = +"", [], @pattern.dup
 
       while true
         match = PatternMatcher.match(_pattern)