This is not an error. If you have detected that relationship identifiers are NULL because you are using RF1 files to update SNOMED CT in your systems SNOMED International strongly recommends that you use Release Format 2 distribution instead. SNOMD International will not distribute or support RF1 files after July 31, 2016.
On Wednesday, 28 October 2015, the IHTSDO General Assembly voted to approve the deprecation of SNOMED CT Release Format 1 (RF1). RF1 had been replaced by Release Format 2 (RF2) as the authoritative distribution format for the SNOMED CT International Edition in July 2011. As part of the deprecation process IHTSDO agreed to continue to deliver RF1 files, converted from the authoritative RF2 release files. As of July 31, 2017 the utility that IHTSDO/SNOMED International used to create the RF1 files will be made available to the public. Also as part of the deprecation process, the relationship identifier column in the RF1 Relationships file was left blank in releases after January 1, 2016, to avoid the possibility of identifier collisions after the conversion utility was made public.
For more information on the deprecation of RF1, please see the deprecation proposal:
The notice of deprecation that was posted as part of the process:
http://www.snomed.org/news-articles/deprecation-notice-snomed-ct-release-format-1-rf1