Calendar Control PRoblem

[Laz]

I need some assistance.

I am getting the error "Conversion from type 'DBNull' to type 'Date' is
not valid." while rendering the edit template column of a calendar
control where the SQL field is null.

Here is the column code:
<ASP:TemplateCo lumn HEADERTEXT="Pla nned Date"
ITEMSTYLE-HORIZONTALALIGN ="Center">
<ItemTemplate >
<%# String.Format(" {0:d}", DataBinder.Eval
(Container.Data Item, "WISP_Upgrade_P lanned_Date")) %>
</ItemTemplate>
<EditItemTempla te>
<ASP:Calendar ID="plannedDate " RUNAT="Server"
SelectedDate='< %# CheckDate(DataB inder.Eval
(Container.Data Item, "WISP_Upgrade_P lanned_Date")) %>'
VisibleDate='<% # CheckDate(DataB inder.Eval
(Container.Data Item, "WISP_Upgrade_P lanned_Date")) %>'/>
</EditItemTemplat e>
</ASP:TemplateCol umn>

As you can see, I tried creating a function that supplies a date value
of Now() if the column is null.

Function CheckDate(Selec tedDate As DateTime)
If IsDBNull(Select edDate) Then
Return Now()
Else
Return SelectedDate
End If
End Function

But IIS never gets to even try the function as the error happens BEFORE
the function is executed.
Please advise.

Thanks