Fix auto complete exception

Fix exception caused by this request:

 http://sapi.unepwcmc-012.vm.brightbox.net/api/v1/auto_complete_taxon_concepts?taxonomy=cms&ranks%5B%5D=KINGDOM&ranks%5B%5D=PHYLUM&ranks%5B%5D=CLASS&ranks%5B%5D=ORDER&ranks%5B%5D=FAMILY&per_page=1000