Hide object class in field selection

Hi All,

I have a scenario in my project where in when I create a wbs element, the cost center should be the required field, I have made the necessary configuration in field selection but when I create a wbs, along with the cost center ,object class has also become mandatory.

I have selected hide for all the object class for wbs in modified and influencing fields, but still when I create wbs the system asks to select object class before saving the wbs created.

regards,

Vineeth