C64 won't let me send my data during Talk

I've got my computer able to talk to the C64 and I'm able to read all the commands and get up to doing the Talk turnaround. But right after I send my first byte of data, the C64 suddenly prints FILE NOT FOUND and stops reading any data. The C64 waits for all 8 bits of the first byte and gives me the handshake at the end, but then just aborts.

Cannot figure out what I'm doing wrong. I've tried 2 different C64s and both do the same thing, and both work fine with a real 1541. The protocol is so simple I can't see what I could be doing wrong since there's not much more to do than sending the bits down the line.


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