FilterExpression for table

[RA]

dim RCodeRows() as DataRow

Filter = "MasterReasonCodeID IN ('CFTExchge','Others','Test','Warranty')"
RCodeRows = DataSet1.ReasonCode.Select(Filter)

In ReasonCode table there are 33 rows. Based on above filter I get 30 rows
because for 3 rows MasterReasonCodeID is null.

Now I would like to do filter as follows:
Filter = "MasterReasonCodeID NOT IN
('CFTExchge','Others','Test','Warranty')"

So I should get 3 rows; instead RCodeRows length shows 0.
Is there anything wrong in my second Filter statement? Are we not able to
use "NOT" for filter expression?

NOTE: MasterReasonCodeID is not a part of the key of table.