I disagree in a very general context.
Mixing different types of results is in general a bad programming style.
Btw, it would be very easy to change the API here with no drawbacks.

Further, returning 0 could also mean that the connection is lost.
So, one has to figure out if 0 means:
1) 0 open trades
2) Connection lost
3) Command not supported

Having to look up various other places what 0 could possibly mean defeates the purpose of pure function call.