Inconsistency in characteristic

Hi,

I have created a characteristic called : CU_INCLUDED_COST, it has three possible values : Labor only, material only or labor and material.

I added the characteristic to a class and i assigned it to a Compatible Unit.

I try to select a value from the drop down list to assign to the compatible unit but i have a message and the value is not saved : "E: Inconsistent values inferred for characteristic Included cost"

Please advise.