Passing Date Range To A Subreport

I am passing a date range to a subreport which is a table that I want to show a list of months in a specific range. I am passing:

FromDateShipedYearMonth - ([Date Shipped].[Month].&[2006]&[4]&[12]
ToDateShippedYearMonth - ([Date Shipped].[Month].&[2007]&[4]&[11])

I have parameters set up in my Main report:


<Subreport Name="subreport2">

<Parameters>

<Parameter Name="FromDateShippedYearMonth">

<Value>= "(" &amp; Code.PreviousYear(Parameters!DateShippedMonth.Value) &amp; ")"</Value>

</Parameter>

<Parameter Name="ToDateShippedYearMonth">

<Value>="(" &amp; Parameters!DateShippedMonth.Value &amp; ")"</Value>

</Parameter>

</Parameters>

And in my Subreport:


<ReportParameter Name="FromDateShippedYearMonth">

<DataType>String</DataType>

<Prompt>FromDateShippedYearMonth</Prompt>

</ReportParameter>

<ReportParameter Name="ToDateShippedYearMonth">

<DataType>String</DataType>

<Prompt>Year - Month</Prompt>

</ReportParameter>

</ReportParameters>

I would like to display data from 12/2006 through 11/2007. Instead I get a list of 1/2006 through 12/2006.

Any ideas?