Key not found error for collection.LookupIn operation of Couchbase

Question

I have the following document in Couchbase:

{
  "id": "ee::brown",
  "type": "ee",
  "firstName": "John",
  "lastName": "Brown",
  "email": "[email protected]"
}

The code below tries to get the lastName field for id="ee::brown"

ops := []gocb.LookupInSpec{
   gocb.GetSpec("lastName", &gocb.GetSpecOptions{}),
   }
getResult, err := collection.LookupIn("ee::brown", ops, &gocb.LookupInOptions{})
if err != nil {
  panic(err)
}

var lastName string
err = getResult.ContentAt(0, &lastName)
if err != nil {
   panic(err)
}
fmt.Println(lastName) 

Running this code generates the following message:

Not Found (KEY_ENOENT)

Running following query in Query workbench retrieves the full document:

select * from mybucket where id="ee::brown"

Also tried setting IsXttra field to true as follows but it did not work either:

ops := []gocb.LookupInSpec{
    gocb.GetSpec("lastName", &gocb.GetSpecOptions{IsXattr: true}),
}

Also could not find definition of IsXattr for GetSpecOption. What is it for? Help please!

Environment: macOS Catalina version 10.15.3; go-sdk : import "github.com/couchbase/gocb/v2" ; Couchbase: Enterprise Edition 6.5.0 build 4960