Color code based on 2 col values

My expression for backgrounfColor:=iif( Fields!ColA.Value<>Fields!ColB,"Red","Green")is returning wrong:Error is:The background color expression for the textbox 'ColA' contains an error:[BC30452] Operator <> is not defined for types Object and Microsoft.ReportingServices.ReportProcessing.ReportObjectModel.FieldHow do get over this.I a trying compare ColA and ColB values and if the values are not equal the color code that row as red if they are equal color code the row as green