diff --git a/lib/csv.rb b/lib/csv.rb
index 89e1a2b..6ffb2c3 100644
--- a/lib/csv.rb
+++ b/lib/csv.rb
@@ -1456,7 +1456,7 @@ def generate(str=nil, **options)
       encoding = options[:encoding]
       # add a default empty String, if none was given
       if str
-        str = StringIO.new(str)
+        str = StringIO.new(+str)
         str.seek(0, IO::SEEK_END)
         str.set_encoding(encoding) if encoding
       else