NAN allele encoding in JSON output

Describe the issue

Presence of a NAN allele results in invalid JSON encoding.

Given the following input:

7 55269675 55269675 C/NAN +

The following output was given (abbreviated)

{
    "transcript_consequences": [
        {
            "variant_allele": -nan
        }
    ],
    "allele_string": "C/NAN"

Additional information

Generally, variants like this should be filtered, but we can't always control the data we're given.
This only appears to affect NAN, not NA, but we haven't fully vetted all combinations.
Both nan and -nan have been seen.

System

VEP version: 108, 110, 111
VEP Cache version: 108, 110, 111
Perl version:
OS: Ubuntu 22.04 (Also seen on OSX
tabix installed:

Full VEP command line

docker run -t --rm {binding} {self.image}:{self.tag} vep \
--everything --json --hgvsg --ga4gh_vrs --offline --no_stats --xref_refseq --verbose
--cache --dir_cache {bound_path} --fasta {path}
--input_file {path} --output_file {path}

Full error message

n/a -- VEP does not error.

Data files (if applicable)

Full output record: {"nearest":["ENST00000454757"],"most_severe_consequence":"intron_variant","transcript_consequences":[{"ccds":"CCDS5514.1","gene_id":"ENSG00000146648","trembl":["Q75MF2_HUMAN","I3WA73_HUMAN","I3WA72_HUMAN","G9MC81_HUMAN","F1JTL6_HUMAN","E9PFD7_HUMAN","C9JYS6_HUMAN","A7VN06_HUMAN"],"source":"Ensembl","given_ref":"C","uniparc":["UPI000003E750"],"used_ref":"C","gene_symbol_source":"HGNC","consequence_terms":["intron_variant"],"gene_symbol":"EGFR","hgnc_id":3236,"protein_id":"ENSP00000275493","strand":1,"impact":"MODIFIER","intron":"26/27","biotype":"protein_coding","swissprot":["EGFR_HUMAN"],"gene_pheno":1,"transcript_id":"ENST00000275493","canonical":1,"variant_allele":-nan,"refseq_transcript_ids":["NM_005228.3"]},{"source":"Ensembl","given_ref":"C","uniparc":["UPI000011F91B"],"gene_id":"ENSG00000146648","trembl":["Q9H3D0_HUMAN","F1JTL6_HUMAN","C9JYS6_HUMAN"],"gene_symbol":"EGFR","consequence_terms":["intron_variant"],"hgnc_id":3236,"used_ref":"C","gene_symbol_source":"HGNC","impact":"MODIFIER","strand":1,"biotype":"protein_coding","intron":"17/17","protein_id":"ENSP00000410031","transcript_id":"ENST00000442591","variant_allele":-nan,"gene_pheno":1},{"gene_pheno":1,"transcript_id":"ENST00000454757","variant_allele":-nan,"protein_id":"ENSP00000395243","biotype":"protein_coding","intron":"26/27","impact":"MODIFIER","strand":1,"gene_symbol_source":"HGNC","used_ref":"C","gene_symbol":"EGFR","consequence_terms":["intron_variant"],"hgnc_id":3236,"gene_id":"ENSG00000146648","trembl":["Q75MF2_HUMAN","I3WA73_HUMAN","I3WA72_HUMAN","G9MC81_HUMAN","F1JTL6_HUMAN","E9PFD7_HUMAN","C9JYS6_HUMAN","A7VN06_HUMAN"],"source":"Ensembl","given_ref":"C","uniparc":["UPI00020655C0"]},{"strand":1,"impact":"MODIFIER","biotype":"protein_coding","intron":"25/25","protein_id":"ENSP00000415559","transcript_id":"ENST00000455089","variant_allele":-nan,"gene_pheno":1,"source":"Ensembl","uniparc":["UPI000050D030"],"given_ref":"C","gene_id":"ENSG00000146648","trembl":["Q504U8_HUMAN","I3WA73_HUMAN","I3WA72_HUMAN","G9MC81_HUMAN","F1JTL6_HUMAN","A7VN06_HUMAN"],"consequence_terms":["intron_variant"],"gene_symbol":"EGFR","hgnc_id":3236,"used_ref":"C","gene_symbol_source":"HGNC"},{"impact":"MODIFIER","strand":1,"biotype":"retained_intron","distance":1180,"gene_pheno":1,"variant_allele":-nan,"transcript_id":"ENST00000485503","gene_id":"ENSG00000146648","given_ref":"C","source":"Ensembl","used_ref":"C","gene_symbol_source":"HGNC","hgnc_id":3236,"gene_symbol":"EGFR","consequence_terms":["downstream_gene_variant"]},{"gene_id":"1956","given_ref":"C","source":"RefSeq","used_ref":"C","gene_symbol_source":"EntrezGene","hgnc_id":3236,"gene_symbol":"EGFR","consequence_terms":["intron_variant"],"protein_id":"NP_001333826.1","impact":"MODIFIER","strand":1,"biotype":"protein_coding","intron":"25/25","variant_allele":-nan,"transcript_id":"NM_001346897.2"},{"gene_id":"1956","source":"RefSeq","given_ref":"C","gene_symbol_source":"EntrezGene","used_ref":"C","consequence_terms":["intron_variant"],"gene_symbol":"EGFR","hgnc_id":3236,"protein_id":"NP_001333827.1","intron":"26/26","biotype":"protein_coding","strand":1,"impact":"MODIFIER","transcript_id":"NM_001346898.2","variant_allele":-nan},{"variant_allele":-nan,"transcript_id":"NM_001346899.2","protein_id":"NP_001333828.1","impact":"MODIFIER","strand":1,"intron":"25/26","biotype":"protein_coding","used_ref":"C","gene_symbol_source":"EntrezGene","hgnc_id":3236,"consequence_terms":["intron_variant"],"gene_symbol":"EGFR","gene_id":"1956","given_ref":"C","source":"RefSeq"},{"given_ref":"C","source":"RefSeq","gene_id":"1956","hgnc_id":3236,"consequence_terms":["intron_variant"],"gene_symbol":"EGFR","used_ref":"C","gene_symbol_source":"EntrezGene","impact":"MODIFIER","strand":1,"biotype":"protein_coding","intron":"26/27","protein_id":"NP_001333829.1","variant_allele":-nan,"transcript_id":"NM_001346900.2"},{"variant_allele":-nan,"transcript_id":"NM_001346941.2","protein_id":"NP_001333870.1","biotype":"protein_coding","intron":"20/21","impact":"MODIFIER","strand":1,"gene_symbol_source":"EntrezGene","used_ref":"C","hgnc_id":3236,"consequence_terms":["intron_variant"],"gene_symbol":"EGFR","gene_id":"1956","given_ref":"C","source":"RefSeq"},{"transcript_id":"NM_005228.5","variant_allele":-nan,"canonical":1,"protein_id":"NP_005219.2","impact":"MODIFIER","strand":1,"intron":"26/27","biotype":"protein_coding","used_ref":"C","gene_symbol_source":"EntrezGene","consequence_terms":["intron_variant"],"gene_symbol":"EGFR","hgnc_id":3236,"gene_id":"1956","source":"RefSeq","given_ref":"C"}],"id":"523693fc-239c-4f65-8448-6b764c7e6840","input":"7\t55269675\t55269675\tC/NAN\t+\t523693fc-239c-4f65-8448-6b764c7e6840","start":55269675,"variant_class":"indel","end":55269675,"seq_region_name":"7","strand":1,"assembly_name":"GRCh37","allele_string":"C/NAN"}

Ensembl / ensembl-vep