Getting 'An Expression of non-boolean type specified in a context where a condition is expected' error using an IIF statement in a WHERE clause

Question

The IIf statement below is throwing 'An Expression of non-boolean type specified in a context where a condition is expected' error.

How do you return a boolean value using an IIf statement within a WHERE clause in SQL server? I tried Casting the result to a bit but it didn't work...

DELETE FROM Assignments
    WHERE AssignedObjectTypeID=@AssignedObjectTypeID AND AssignedObjectID=@AssignedObjectID 
    AND TargetObjectTypeID=201 AND IIf(@Entity Is Null,1,0)