_search endpoint returns a 0 response when passing in URI paramters

When running a curl with no parameters in _search eg:

curl -XGET 'http://elasticsearch.url:9200/an_index-*/_search

I am getting the desired results, however when I do a parameterized uri, ie:

curl -XGET 'http://elasticsearch.url:9200/an_index-*/_search?pretty=true&size=10

I get a curl: (52) empty reply from server.

Note that both calls are being called externally. If I call them within the elasticsearch instance ie. localhost, I get results on the second curl command.

Is this a configuration issue or a general bug? If it is the former, what is the correct configuration to enable parameterized uris from an external host.