LOCATE function fails in a very specific way

[astefan]

SELECT LOCATE(CASE WHEN FALSE THEN NULL ELSE LEFT(first_name, 1) END, first_name) > 0 AS x FROM test_emp

fails with

{
  "error" : {
    "root_cause" : [
      {
        "type" : "illegal_argument_exception",
        "reason" : "expected [3] children but received [2]"
      }
    ],
    "type" : "illegal_argument_exception",
    "reason" : "expected [3] children but received [2]"
  },
  "status" : 400
}

No stack trace in logs...
Failed to simplify the failing test more than SELECT LOCATE(CASE WHEN FALSE THEN NULL ELSE 'x' END, first_name) > 0 AS x FROM test_emp, it seems it really needs to have a CASE WHEN that goes on a FALSE branch.

[elasticmachine]

Pinging @elastic/es-search (:Search/SQL)

[astefan]

master (8.0.0): dd3aeb8
7.x (7.6.0): astefan@4dc83a7
7.5 (7.5.1): e14d00c