Invalid error for semantically correct query

[armaneous]

• What version of Dgraph are you using?
  dgraph:latest

• Have you tried reproducing the issue with latest release?
  Yes

• Steps to reproduce the issue (command/config used to run Dgraph).
  my_id is indexed on exact in schema. Performing this mutation:

{
  set {
    _:test <pred_one> "one" .
    _:test <test> "" .
    _:test <my_id> "123" .
  }
}

• Expected behaviour and actual result.
  Query should return no results:

{
  t as var(func: eq(my_id, "123")) @filter(has(test)) @cascade {
    p as pred_test
  }
  query(func: eq(my_id, "123")) @filter(uid(t) AND eq(val(p), "my test")) {
    expand(_all_)
  }
}

Instead, this error appears:
: Expected a valid value map. But got empty.

Modifying the query to be of this format returns the expected no results found:

{
  t as var(func: eq(my_id, "123")) @filter(has(test)) @cascade {
    p as pred_test
  }
  query(func: uid(t)) @filter(eq(my_id, "123") AND eq(val(p), "my test")) {
    expand(_all_)
  }
}

[danielmai]

@srfrog Can you take a look at this?