IfcTimeSeries.HasExternalReference inverse lower bound 1

I was working on some sample data and realized that the lower bound for HasExternalReference on IfcTimeSeries is 1, so an external reference for time series is required to exist. Is that intentional?

> HasExternalReference: SET [1:?] OF IfcExternalReferenceRelationship FOR RelatedResourceObjects
>
> Reference to an external reference, e.g. library, classification, or document information, that is associated to the IfcTimeSeries.

http://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcTimeSeries.htm