Operator '&' is not defined for string

I am getting this error on my Expression to control whether my report, letter prints an Address Line 2 or not. Here's the expression...

=IIf(Fields!Address_Line_2.Value = Nothing, Fields!City.Value & ", " & Fields!State.Value & " " & Fields!Zipcode.Value, Fields!Address_Line_2.Value)

It is working ok...just getting a Warning Message.

Any help?

Thanks for your review and am hopeful for a reply.