How to make the generate_series to generate the letter series?

Hi team,
when I study the generate_series function, I found that it can not only generate the number series but also can generate the date series.(ref: https://www.postgresql.org/docs/current/functions-srf.html)
That means I can make the generate_series to generate the letter series.

So I try this command (I guess I can follow the ascil code rule to complete this work):

# SELECT * FROM generate_series('a'::int, 'z'::int, 1);

But the postgres reply me a error, because the 'a' can't translate into 'integer' type.

Can someone provide me a better solution?
Thanks in advance!

Yours,
Wen Yi