You should rewrite the query so there is only one parameter.

SELECT Value = CASE WHEN Arg = '19000101' THEN GETDATE() ELSE Arg END FROM (SELECT Arg = ?) a

or even

DECLARE @arg DATETIME;

SET @arg = ?;

SELECTValue = CASE @arg WHEN '1900-01-01' THEN GETDATE() ELSE @arg END