Passing null parameters to strongly typed datacontext

OK, next step.* I've now got a strongly typed DataContext which I've associated with a stored procedure in my SQL Server 2005 database.* This SP does allow for nulls to be passed in for various parameters; in fact passing in nulls for some of the parameters will result in more data being returned, which is exactly what I want.* However, I don't know how to pass in nulls for the parameters of the method that is my SP associated with the LINQ to SQL result class.* I'll do something like the following to declare variables to pass to the method of the result class:

<b>string firstName, lastName, socialSecurityNo;
DateTime? dateOfBirth;

firstName = "Fred";
lastName = "Flintstone";

MyStronglyTypedDataContext dc = new MyStronglyTypedDataContext();</b>

System.Data.LINQ.ISingleResult result = dc.spCheckData(firstName, lastName, socialSecurityNo, dateOfBirth);

As you can see, I've left the <b>socialSecurityNo</b> and <b>dateOfBirth</b> values alone, which will mean that <b>socialSecurityNo</b> will likely equal string.Empty and <b>dateOfBirth</b> will be null.* These are allowed for in the SP, but running this results in nothing being returned.* I know this is not true, because if I run the same query in SSMS I get results.* So, how do I assign values to those variables so that when the call to spCheckData() occurs it will pass in null values to the SP?