MalformedSelectorError

The Cause

A value was used where a Selector was expected, but it is not one.

The grammar is:

METHOD SP path [ SP "(" requestMediaType ")" ] SP "->" SP status [ SP "(" responseMediaType ")" ]

GET /launches -> 200 (application/json)
POST /astronauts (application/json) -> 201 (application/json)
DELETE /astronauts/{id} -> 204

What to do

The error suggests a canonical spelling when the value is nearly a Selector — a lowercase method, a missing leading /, or a zero-padded status:

A selector spells its method in uppercase, spells its path with a leading "/"
and spells its status without leading zeros.
Did you mean "GET /launches -> 200"?

The most common genuine mistake is omitting the -> status part entirely: a Selector always names a status, because a status is part of what identifies a Transaction.

Note the media part appears whenever the description declares a media type — not only when there is a body.