How to prevent bottom mattress edge from getting pushed under metal bed frame?

I just got a new mattress and metal bed frame, and whenever I sit on the bottom edge of the bed to get off of it, the bottom edge of the mattress falls through the gap in the metal bed frame and gets pushed under the edge of the frame. What is the best way to prevent the mattress from falling through the gap every time there is any weight put on that area? Thanks so much in advance for any advice!