The IMSUM function has a claim about applications accepting zero parameters for this function. However, it has a production that includes "ComplexSequence," which as far as I can determine isn't defined in the draft.

I suspect it means a set of complex numbers but we don't say that.

If it is one or more complex numbers, by definition, then applications cannot allow zero complex numbers as a value, it would be a parameter type error and therefore zero complex numbers would return an error.