Computed protein residue variants from Ensembl appear as 'null'

Noticed after:
1. retrieve example Ensembl ENSG00000157764
2. show ENSEMBL crossrefs.
Browse to around column 476 of protein panel - see
p.Gly469null - takes you to
http://www.ensembl.org/Homo_sapiens/Variation/Explore?r=7:140781103-140782103;v=rs121913357;vdb=variation;vf=23761128

This variant is an N (ie ACGT). Perhaps the null arises because at least one of the codons is invalid ? (notes in variant indicates stop codons have been observed).