max_by/min_by specification disagrees with the examples

[sliiser]

The max_by and min_by specification declares that the 2nd argument is an expression that returns either a number or a string.

expression->number|expression->string

However, they both have an example that specifically contradicts this showing that strings are not allowed. Which is it?

Expression	Result
max_by(people, &age_str)	<error: invalid-type>
min_by(people, &age_str)	error: invalid-type