UMLR — Multiplicity of Operation::returnResult() should be [0..1], i.e. not multivalued

There is at the most only one return parameter allowed for an Operation due to the existing constraint "at_most_one_return". Why is the returnResult() operation multivalued? It should be [0..1], not [0..*].

Instead of an empty set, it could return null if there is no return parameter, similar to lower(), type() and upper(). But the stated body for those operations is defined in terms of returnResult() returning a set. Would that have an impact?