Transaction COR2 release userexit , Batch number should be blank

Hi,

     My requirement is , when i release process orderf rom COR2 tcode, the batch number should come us blank. I used EXIT_SAPLV01Z_002 from function module VB_NEXT_BATCH_NUMBER and made the batch number as blank. But after that exit, there's a condition in VB_NEXT_BATCH_NUMBER saying if batch number is blank then throw an error. Is there any way, i can get the batch number as blank after releasing the process order.

Thanks,

Vandana