How is this move illegal? I'm black and it's my turn. I'm playing in the Arena chess player against Stockfish.
This is my current board:

[fen ""]
[startply "31"]

1. e4 d5 2. exd5 e6 3. dxe6 fxe6 4. Nf3 Nf6 5. Bc4 c6 6. O-O Bc5 7. d4 Bb6 8. Re1 O-O 9. Nc3 Nbd7 10. Ng5 Ne5 11. dxe5 Qxd1 12. Nxd1 Ng4 13. Nxe6 Bxf2+ 14. Nxf2 b5 15. Bb3 Nxf2 16. Nc7+


I want to play Rb8:

1rb2rk1/p1N3pp/2p5/1p2P3/8/1B6/PPP2nPP/R1B1R1K1 w - - 2 17 


but I keep getting errors telling me that's illegal.

HOW IS THIS ILLEGAL??
It turns out I was in check:&nbsp;