problem with Rowdeleting event

Hi,

Before deleting a record in a gridview, i want to check the value of a
particular field.

I do this with this code (vb.net):

Protected Sub GridView1_RowDeleting(ByVal sender As Object, ByVal e As
System.Web.UI.WebControls.GridViewDeleteEventArgs) Handles
GridView1.RowDeleting
Dim myval As String
myval = e.Values.Item("myfield").ToString
......
if myval..... then
e.cancel
end if

This works only when the field is not empty. If it's empty, i get the error:
""Object reference not set to an instance of an object"

Any way to avoid that error?
Thanks
Bart