I don’t think any error occurs. This is because at modbus protocol level there are no concepts such as int64 or even float32. There are just 16 bit registers (input and holding), 1 bit coils, and 1 bit discrete inputs.
Naturally you would get an configuration error if you try to use unknown valuetypes such “foobar” or “int64”.
So the binding can poll the data just fine. Constructing the number just needs some work as mentioned above. Arithmetic is following similar logic @alexopenhab explains above.
Best
Sami