Puzzle Error

I was playing a puzzle when I came across something that I just couldn't comprehend. The puzzle code is posted below. After 43... Bf3+ you apparently can't play the move 44. gxf3. I've been staring at it for conservatively the last 20 minutes and I just can't comprehend it. Can someone help me out here?

Puzzle Code: #4x7Wa

I now see the error, I thought the white pawn could take backward. This is why I'm hardstuck 800, isn't it?