syntax error with alter type

I’m running Pg 9.6.5 if it matters…

I’m trying to drop a value from an ENUM (type) and it seems like I’m following the fine manual yet I still get an error. For example:

nms=# create type alphabet as enum ('a', 'b', 'c', 'd');

CREATE TYPE

nms=# alter type alphabet drop attribute if exists 'c';

ERROR:  42601: syntax error at or near "'c'"

LINE 1: alter type alphabet drop attribute if exists 'c';

                                                     ^

LOCATION:  scanner_yyerror, scan.l:1086

What am I doing wrong? The goal is to get rid of the ‘c’ value from the enum. Yes, you can assume I’ve already removed of all the ‘c’ values in the table where it’s used.

Or does that statement not do what I think it does and I have to do the “create new type, change the table to use the new type, drop old type, rename new type to old type” routine?

Thanks,

Kevin