500kHz WSPR - when I forgot the ATU!

Having used my 5m long vertical on 40m to work OY9R on QRP SSB this afternoon, I went back onto 500kHz WSPR but completely forgot to connect the ATU, so the antenna was connected directly to the transverter with no matching at all! I only realised this when I got a report from M0BMU some 20dB below that normally received (at -26dB S/N). I've since reconnected the 500kHz ATU and the 500kHz signal is back to normal. The IRF510 survived the gross mismatch.