Couchbase View empty, N1QL returns results -

i using travel-sample provided editor perform tests.

when map (below) using doc.type=="landmark" or doc.type=="route", results, when user "airline" type, no results view. rather puzzling.

function (doc, meta) { if (doc.type && doc.type=="airline") {     emit(doc.id, doc.type);   } } 

i can't quite figure out why returns empty view. although following equivalent n1ql query returns results :

select id `travel-sample`  type = "airline" limit 10