Animating focal length on the fly?

I can animate a camera’s focal length by right-clicking on the Focal Length control and choosing “Insert Keyframe”. However, how do I record changing the focal length while the animation is playing back?

I’ve tried to add Focal Length to the keying set, by clicking “Add to Keying Set”. However, I get a “Cannot add property to built in keying list” error when I try to do so. (I’m on Blender 2.75.)

Am I missing something?