Export relationship custom data --> generates error

The export select data form allows core data of a related entity to be exported. It also appears to allow the custom data of the relationship, but not the related entity, to be exported. Selecting relationship custom data for export will cause the export to fail with the error "DB Error: value count on row".

Of the SQL generated that causes this error - the field list omits the relationship custom data fields, whereas the values list includes them as blank. There appears to be no support in code for the custom data fields of relationships to be exported. On the other hand, there is support in code for the custom data of the related entity, but these cannot be selected from the UI.

A fix would be to correct the mapping that lists the relationship custom data fields to list the related entity custom data fields.
Alternatively (and ideally) support the export of the relationship custom data and display is the mapping list the related entity's custom data fields.