INSTEAD OF Trigger - Error Inserting Into NOT NULL Field

I've just noticed some strange behavior that seems like a bug to me.
It's much easier to follow an example of it that to outright explain it, so here goes.

I have a table defined with a NOT NULL constraint on a column and a default clause:
-- DROP TABLE TestTable
CREATE TABLE TestTable ( TestField0 varchar(10), TestField1 varchar(10) NOT NULL DEFAULT ('a') )

I have a view defined on the table, in this example case, the view just mirrors the table one to one:
-- DROP VIEW TestView
CREATE VIEW TestView as SELECT TestField0, TestField1 FROM TestTable

So far so good, if I run this statement, it works as I would expect and inserts the value and the default goes into the other field:
INSERT INTO TestView (TestField0) SELECT 'test'

Now... If I add an INSTEAD OF trigger to the view, and have it perform the insert for me, I get an error with the same insert stmt:
-- DROP TRIGGER TestTrigger
CREATE TRIGGER TestTrigger ON TestView
INSTEAD OF INSERT AS
BEGIN
INSERT INTO TestTable (TestField0, TestField1)
SELECT TestField0, COALESCE(TestField1, 'X')
FROM inserted
END

Notice the trigger will ensure that a null value cannot be inserted into TestField1. If I run this insert stmt though I get an error:
INSERT INTO TestView (TestField0) SELECT 'test'

Server: Msg 233, Level 16, State 2, Line 1
The column 'TestField1' in table 'TestView' cannot be null.


Am I missing something or is this a bug?
Thanks