Draw claim rejected

In this game: https://www.redhotpawn.com/chess/chess-game-analysis.php?gameid=15206025

I am white in this game and I have just made move 41 which produces a threefold repetition (same position after white's move 37 and 39). I claimed a draw as I made the move but it was rejected. Any idea why?

@stillropey
Hold on I've just seen something. My 37th move was 37. f4.
If there had been an opponent pawn in position they would have been able to capture en passant (there wasn't). So I wonder if it has rejected on that basis.

@stillropey
My opponent has now claimed on his next move. Strange though.

@stillropey said
@stillropey
My opponent has now claimed on his next move. Strange though.

I have never known the software to be at fault on this site.
The coding has been in place for years and unlikely to have been corrupted.
If it had developed a fault,there would be dozens of messages on here

@venda
Like I said, the only reason I can think was that the in the first position the pawn had been advanced two spaces. The software might be programmed to consider that to be a different position owing to en passant possibilities even though there were no opponent pawns that could have captured.

You say that it hasn't happened before but this is a relatively rare situation so might not have come up. Even if it had happened, usually threefold draws are more or less consensual and if the opponent claims (as in my case) then it might have been mentioned.