500 internal server error calling api/sequence

I tried a few, here seqence checksum off ENST00000311936.3:

```
curl -X GET --header 'Accept: application/json' 'http://tark.ensembl.org/api/sequence/?seq_checksum=5146728D5523DF9C9E36CD799192BA2245904F43'
```

Errors with 500 and return HTML instead of JSON:

```
    <title>Error: 500 | EMBL’s European Bionformatics Institute</title>
```
