Before we answer, here's a
link to an easy insurance purchase based upon the type of vendor that you are.
It sounds like you've tempted fate and come out on top, but luck is a fickle mistress. You're never asked for your car insurance either, until you have an accident. Of course, then you'll wish you had it.
Any business has an obligation to have liability insurance. If you don't have insurance, you are shifting your liability to others, and in the event world that's the event host (your client) and the venue (us). And we're not willing to accept the liability incurred by others anymore than they should expect to incur ours. And your client will surely feel the same.
Short answer - you need an annual insurance policy. No exceptions.