.net - LINQ Null Date Comparison In Dataset?

I have a DataSet where I need to filter the collection where one of the date columns is not null.

How do you term this in LINQ?

This is what I have so far but there is a compiler error on the w.CompletedDate field in the Where clause.

Dim completed = From ins In InspectionDataset.Inspection.AsEnumerable _
Where (Function(w) w.CompletedDate IsNot Nothing) _
Order By ins.Field(Of DateTime)("UpdatedDate")

I have also tried it the below way but the compiler will not allow the DBNull comparison.

Dim completed = From ins In Inspection.AsEnumerable _
Where ins.Field(Of DateTime)("CompletedDate") <> DBNull.Value _
Order By ins.Field(Of DateTime)("UpdatedDate")