function json_populate_record(..., text) does not exist

I went by the example from the docu which went fine:

select * from json_populate_record(null::x, '{"a":1,"b":2}')

But my self-constructed JSON simplified similar to this did not work :-(

-- p_some_num of type int
select * from json_populate_record( null:my_record_type, '{"a":'||p_some_num||'',"b":2}' )

resulting in:

ERROR: function json_populate_record(my_record_type, text) does not exist

I should have read more carefully and was not aware of the implicit text to json conversion in the example. After some fiddling around with what was wrong, of course the following works:

select * from json_populate_record( 
  null:my_record_type, ('{"a":'||p_some_num||'',"b":2}')::json )

StackExchange Database Administrators Q#123053, answer score: 3