Index is not used - parameterized queries

When this query is used, couchbase is performing full data scan (index is not used)

var query = n1qlQuery.fromString("SELECT LOWER(e1.color) as color, e2.* FROM xxx e1 JOIN xxx e2 ON KEYS e1.person_id WHERE e1.type = $1 AND e1.person_id = $2"); bucket.query(query, ["alert", personId], function(err, person) {} )

But when query is structured in this way, index is used

var query = n1qlQuery.fromString("SELECT LOWER(e1.color) as color, e2.* FROM xxx e1 JOIN xxx e2 ON KEYS e1.person_id WHERE e1.type =\"alert\" AND e1.person_id = \""personId"\"");
bucket.query(query, function(err, person) {} )