Do I have an obligation to answer honestly when a credit card company asks me to enter my income?

I logged into my credit card company’s bill payment web site – before I could pay, they displayed a screen asking me to enter my annual income.

Ostensibly this is to gauge if they will increase my credit limit.

Do I have to answer? Can I just make up a number (correct or not)? What’s the penalty for not answering or inventing some number?

The way I see it, I pay my bill on time every month. That’s all they need to know.

What’s my obligation?