SSRS Text box expression in Report Header

Hi Experts,

I'm trying below expression under Text box (Report Header)

= IIf( IsNothing(Parameters!vCp_ID.Value) OR Parameters!vCp_ID.Value = "", "All", Join(Parameters!vCp_ID.Value,", ") )

It gives me error message as:- 

Warning	1	[rsRuntimeErrorInExpression] The Value expression for the textrun ‘Header_Cp_ID_Value.Paragraphs[0].TextRuns[0]’ contains an error: Overload resolution failed because no Public '=' can be called with these arguments:   'Public Shared Operator =(a As String, b As String) As Boolean':     Argument matching parameter 'a' cannot convert from 'Object()' to 'String'.	C:\Users\abc1\Documents\Visual Studio 2008\Projects\Report Project1\Report Project1\ClaimsLevel.rdl	0	0

Any help what I'm doing wrong? Because of this is it printing "#Error" while previewing the Report.

If I remove IIF condition and place only:- 

Join(Parameters!vCp_ID.Value,", ")

than everything is working fine.

Thanks in advance

Kumar