NullReferenceException when retrieving values from LINQ to DataTable - linq

Question

I have an application that is loading data into a database in a background task. The values are retrieved from a datatable using LINQ, manipulated and stored in a new datatable. I'm getting a NullReferenceException error and I can't figure out why. I'm checking to see if the value coming from the LINQ query is null, and if it is, the variable is assigned properly. If, however, the LINQ query contains a value, I get a NullReferenceException "Object reference not set to an instance of an object" error which really makes no sense to me. What am I doing wrong?
Dim pin = (From u In partdata.AsEnumerable() _
Where u.Field(Of String)("PART_GRP") = sPartType And
u.Field(Of String)("INVT_TYPE").Contains("A")
Select oid = u.Field(Of String)("ID"), Position = u.Field(Of Integer?))
Dim pout = (From u In partdata.AsEnumerable() _
Where u.Field(Of String)("PART_GRP") = sPartType And
u.Field(Of String)("INVT_TYPE").Contains("B") Or u.Field(Of String)("INVT_TYPE").Contains("C")
Select oid = u.Field(Of String)("ID"), Position = u.Field(Of Integer?))
If pin.Count > 0 And pout.Count > 0 Then
For i = 0 To pout.Count - 1
ioidID = pout(i).oid
' Parts In
If pin(i).Position Is Nothing Then ' When "Position" is Null, it works and "" is assigned. When "position" is not null, I get a NullReferenceException error on this line.
spinPosition = ""
Else
spinPosition = pin(i).Position
End If
...

Answer 1

Based on the comments to the question, it appears that pin(i) is Nothing. This would cause the exception when you try to dereference it here:
pin(i).Position
Note that you define pin and pout as two separate collections, unrelated to one another. You check that they both contain at least one value:
If pin.Count > 0 And pout.Count > 0 Then
But you never check that they contain the same number of values. You assume this here:
For i = 0 To pout.Count - 1
If pout ever contains more values than pin, your code will fail.