Sounds like an ANSI_NULLS setting thing...  Some sample code:

DECLARE @ClosedStuff TABLE (InspectionClosed DATE NULL);

INSERT INTO @ClosedStuff

VALUES (NULL),(NULL),('19000101'),(GETDATE())

SELECT * FROM @ClosedStuff

SET ANSI_NULLS ON

SELECT SUM(CASE InspectionClosed WHEN '1900-01-01 00:00:00.000'...