What to do when an arm can keep up?

Hi everyone! When you are doing biceps curls, what do you do when one arm can't keep up with the other one? My left arm can't do more curls, but my right one is still good to go...

I just started working out again since 2 years ago. I'm making my own workouts following Jeff's videos until I get enough money to buy one of his workouts maybe Ax-1