ERROR: type "myrowtype" does not exist

I was reading the json_populate_record function from the official doc https://www.postgresql.org/docs/10/static/functions-json.html#FUNCTIONS-JSON-PROCESSING-TABLE <https://www.postgresql.org/docs/10/static/functions-json.html#FUNCTIONS-JSON-PROCESSING-TABLE>. The doc mentioned examples are giving me errors

psql (11.0, server 10.5)
Type "help" for help.

aruprakshit=# select * from json_populate_record(null::extype, '{"x":"foo", "y":43, "q": true}');
ERROR: type "extype" does not exist
LINE 1: select * from json_populate_record(null::extype, '{"x":"foo"...
^
aruprakshit=# select * from json_populate_record(null::myrowtype, '{"a": 1, "b": ["2", "a b"], "c": {"d": 4, "e": "a b c"}}');
ERROR: type "myrowtype" does not exist
LINE 1: select * from json_populate_record(null::myrowtype, '{"a": 1…

What does null::extype, null::myrowtype mean here ?

Thanks,

Arup Rakshit
ar(at)zeit(dot)io