In my experience this just makes them lose confidence in you and the company. So when it eventually is right, they're resistant. Worst case you lose the contract.
If you just fill in the gaps yourself, your software doesn't work. Software that doesn't meet the requirements is legitimately useless. The problem is, most of the requirements are in the customer's head, and you have to squeeze them out. Most of them are unconscious, they might not even know them.