Before DROOLS-5438 the behaviour of sum() was not correct, since it was

sum([]) = 0

while DMNv1.3 specified that ought to be:

sum([]) = null

While DROOLS-5438 fixed the correct result (null value) it was emitting an Error which was never required by the spec, since it mentions:

parameter:

list of 0 or more numbers (...)

result:

or null if list is empty