Error On Trigger When Doing Multiple Row Update?

I have the following trigger that updates a couple test fields to null when they are 1/1/1900, works great on inserts, and one line updates:

CREATE TRIGGER UpdateDate
ON test
AFTER INSERT, UPDATE
AS
UPDATE Test
SET
[CheckDate] = CASE WHEN [CheckDate] = '19000101' THEN NULL ELSE [CheckDate] END,
[CheckDate2] = CASE WHEN [CheckDate2] = '19000101' THEN NULL ELSE [CheckDate2] END
where AutoID = (select AutoID from inserted)

However, when trying to do a multi line update statement, I get the following error:

Msg 512, Level 16, State 1, Procedure UpdateDate, Line 7

Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.The statement has been terminated

How can I get around this?