Date Report Parameter Not Returning a Value (AX 2012)

I have created a RDP report with Contract Class containing 4 parameters. While 3 of the parameters work perfectly, the 4th parameter, InvoiceDate, is not returning a value. I have compared my parm method to other date-based parm methods and it matches syntatically. See below:

[ DataMemberAttribute("InvoiceDate"), SysOperationDisplayOrderAttribute('1')]
public InvoiceDate parmStartDate(InvoiceDate _startDate = startDate)
{

startDate = _startDate;
 return startDate;
}

In my processReport method in the DP class I retrieve the parameter the same way I retrieve the other 3 parameters, this InvoiceDate parameter however I cannot get to return a value. 

dataContract    = this.parmDataContract();
 startDate       = dataContract.parmStartDate();

I've ensured my processReport code works by hard coding an InvoiceDate value, that works. So it must be my parm method. What the heck am I doing wrong?

Thanks so much for your help!