what's the call?

[eastcoastref]

I've never seen something like this happen, but for some reason it popped into my head. It's unlikely to happen, but certainly possible. I'm wondering what the correct call would be.

Here goes:

A1 drives toward the basket while B1 rushes in from opposite side of the floor. A1 misses his layup, but the shot is quickly tipped in by A2. Just after the tip-in, but before A1 reaches the floor, B1 barrels into A1, committing a foul.

What would the proper call be? I've already asked 3-4 people and have gotten 3-4 different answers.