The VM Emulator expects all functions to have a name in the form
ClassName.funName. If you have functions, which do not conform to that, the emulator will throw that exception and basically crash.
Shameless plug: I have a fork of the original sources, which fixes several known bugs (including this one) and adds a few features. You can check it out at
https://github.com/itoshkov/nand2tetris-emu/releases.