Now let me make one thing perfectly clear. There is no ruining here. And definitely no cumming - I shouldn't even need to say this!
If you 'accidently' ruin you should eat your disgusting mess and then close the tease.You will be expelled with immediate effect and sent back to your Mistress, who I'm sure will enjoy punishing you for your lack of control and for wasting her money.
You see, my services are rather expensive - and I don't give refunds!
So unless you want to endure a beating like you've never suffered before, I suggest you do everything you can to make sure you make it through to the end, at least of the first day...