Why is my CASE expression non-deterministic?

I am trying to create a persisted computed column using CASE expression:

ALTER TABLE dbo.Calendar ADD PreviousDate AS 
case WHEN [Date]>'20100101' THEN  [Date]
    ELSE NULL
    END PERSISTED

MSDN clearly says that CASE is deterministic, here

However, I am getting an error:

Msg 4936, Level 16, State 1, Line 1 Computed column 'PreviousDate' in table 'Calendar' cannot be persisted because the column is non-deterministic.

Of course, i can create a scalar UDF and explicitly declare it as deterministic, but is there a simpler way around this? I am already in the middle of getting the latest service pack. Thanks.