Proper use of DBNull

Proper use of DBNull

Can someone inform me of the proper use of DBNull? I'm developing an MVC3
project which has this query in the controller.
I have a linq query that returns a field that I know has no value on the
first cycle:
var qryGetBirthdate = (from c in db.Customer
select new
{
c.Birthdate
}
).FirstOrDefault();
In the database, Birthdate is a Datetime type which is null.
The check I do for this is wrong:
if ( !DBNull.Value.Equals(qryGetBirthdate.Birthdate))
{
DateTime? dob = qryGetBirthdate.Birthdate;
}
I've tried variations from other blogs but can't find the answer.
Basically what I want is to catch the fact that
'qryGetBirthdate.Birthdate' doesn't exist yet. All I'm getting is the
exception that the object isn't set to a reference which I know.
What am I doing wrong?