Petit Modem Error.

Okay, so I just started using Petit Modem, and I was trying to send a sprite from my PC to my Old 3DS. I set the speed to 300 Bytes per Second, and everything was going smoothly. However, there was an "illegal function call" at the end when it was trying to save my file. Is there anything I can do to fix this? What am I doing wrong?

I figured out the problem. My files had lowercase letters in their names, and therefore could not save on SmileBASIC's engine.

My files always have lower-case letters and PM just converts them to upper-case.