BOSTON -- If you are like a lot of people, pop-up ads that interrupt online activities frustrate you. But consumers should be especially careful to avoid a new kind of pop-up, television station WCVB reported.

The station said these ads are more more devious than others. They are a sales gimmick, but that isn't immediately apparent because they initially promise you something for nothing. Then, there's a catch.

The pop-up says you're an instant winner and can choose a free $50 gift card from one of several well-known companies. At that point, the company asks for your e-mail address.

At first glance, it looks like a pretty good deal -- like a contest you were randomly picked for and won.

But when you follow the directions on how to collect your winnings, another screen comes up explaining that in order to get the gift, you have to buy something. Actually, you have to sign up for a Discover or Chase platinum credit card and activate it by making a purchase.

Tiny print at the bottom of the ad reads, "To receive your $50 gift card you must complete a sponsored offer." That isn't free.

According to the station, the offer is illegal in Massachusetts.