SQLDataSource And Null Values

I have a sproce that accepts null for one of its parameters I can execute the sproce and enter null and it works fine, it returns all rows. When I try doing this with my GridView and the SQLDataSource it does not work. I need some help in understanding how the SQLDatasource wants a null. Here is what the parameter row of the SQLDataSource looks like.

<asp:ControlParameter ControlID="EnteredByText" DefaultValue="Null" Name="EnteredBy" PropertyName="Text"

Type="String" ConvertEmptyStringToNull="true" />

In my sproce I have setup the parameter as follows;

@EnteredBy Nvarchar(50)=Null

In my WHERE Clause I have:

WHERE (tblClient.EnteredBy = @EnteredBy OR @EnteredBy IS NULL)