That can be tricky. Venture funded companies that are totally legit and run by competent people sometimes don't want to use a vendor's form. That's not really a red flag...
I would first read the agreement they want you to sign though. If it's not bad and you can live with it, it's sometimes worse it to compromise. Some of the bigger clients cannot be flexible on contracts.
Of course, one important thing is too decide how money you're willing to risk. If you're not willing to risk more than two week's worth of work, then bill them weekly so that you can act quickly in case they decide to stiff you.