@mumblebaj
As of version 2.24, Somebody added to spit this error message out when the 2nd parameter is not a callback function.
However, the following codes correct the missing parameter anyway, so it is unnecessary to regard it as ERROR, unless that self-correction logic is removed.
Even if there is a plan to drop out that correction logic, it’s still better to flag it as a “WARN” rather than an “ERROR”, I think so.
Anyway, the error message is just annoying, and not related to real “ERROR” . You can ignore it.