错误:没有唯一的约束匹配给定的键参考表"bar"

尝试在Postgres 9.1中创建这个示例表结构:

CREATE TABLE foo (
name        VARCHAR(256) PRIMARY KEY
);


CREATE TABLE bar (
pkey        SERIAL PRIMARY KEY,
foo_fk      VARCHAR(256) NOT NULL REFERENCES foo(name),
name        VARCHAR(256) NOT NULL,
UNIQUE (foo_fk,name)
);


CREATE TABLE baz(
pkey        SERIAL PRIMARY KEY,
bar_fk      VARCHAR(256) NOT NULL REFERENCES bar(name),
name        VARCHAR(256)
);

运行上面的代码会产生一个错误，这对我来说没有意义:

NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "foo_pkey" for table "foo"
NOTICE:  CREATE TABLE will create implicit sequence "bar_pkey_seq" for serial column "bar.pkey"
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "bar_pkey" for table "bar"
NOTICE:  CREATE TABLE / UNIQUE will create implicit index "bar_foo_fk_name_key" for table "bar"
NOTICE:  CREATE TABLE will create implicit sequence "baz_pkey_seq" for serial column "baz.pkey"
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "baz_pkey" for table "baz"
ERROR:  there is no unique constraint matching given keys for referenced table "bar"

********** Error **********


ERROR: there is no unique constraint matching given keys for referenced table "bar"
SQL state: 42830

有人能解释为什么会出现这个错误吗?