is it good idea to sign a contract which has a penalty clause for not meeting zero critical defects goal?

I work in the software field as a vendor prviding software solutions to client. After investing in several months of development and testing effort for a project, they are insisting that just before go-live deploy decision is made, they will do dry run and do final verification. if still there are any critical defects found during final verification, supplier (meaning I) has to bear the cost of any further schedule slippage. Is this realistic or advisable to accept ?