changing constraints

I tried adding a constraint thus:
de4=> ALTER TABLE genus ADD CHECK(gender = 'masculine' || 'feminine');
But get the msg:
ERROR: AlterTableAddConstraint: rejected due to CHECK constraint $2

de4=> \d genus
Table "public.genus"
Column | Type | Modifiers
----------------+-----------------------+-----------
genus_name | character varying(20) | not null
gender | character varying(10) |
cas_gen_number | integer |
family_name | character(7) |
Indexes: genus_pkey primary key btree (genus_name)
Foreign Key constraints: $1 FOREIGN KEY (family_name) REFERENCES
family(family_name) ON UPDATE NO ACTION ON DELETE NO ACTION

I cant see a $2 constraint so why am i getting the error msg?

Many thanks Dave