Transact SQL :: Adding Constraint For Business Logic

ALTER TABLE [dbo].[bkrm_lendcoll] ADD CONSTRAINT
     ReqdCovgAmt_constraint33 CHECK     
( case 
  when CovgAmt = 0 and  ReqdCovgAmt = 0 then 1
  when CovgAmt = 0 and  ReqdCovgAmt = 1 then 1
  when CovgAmt = 1 and  ReqdCovgAmt = 0 then 1
  when CovgAmt = 1 and  ReqdCovgAmt = 1 then 0
end =1
)

This is my first attempt to add a constraint for business logic.  The desired behavior is that the two columns together have the same behavior as a radio button.  (one can be true, the other true, both can be false, but both cannot be true.) I get this error when I attempt to update it.

The ALTER TABLE statement conflicted with the CHECK constraint "ReqdCovgAmt_constraint33". The conflict occurred in database "Std", table "dbo.lendcoll".

So, basically my question is, when you have two bit columns and want them to have the truth table such as described, how can I set a Check constraint to enforce this?