Can't add foreign key constraint to table
<pre><code>CREATE TABLE CUSTOMER
(
    CNUM VARCHAR(25) NOT NULL,
    CNAME VARCHAR(75) NOT NULL,
    CTYPE VARCHAR(20) NOT NULL,

    CONSTRAINT CUSTOMER_PK PRIMARY KEY(CNUM),
    CONSTRAINT CHECK_CTYPE CHECK(CTYPE IN('INDIVIDUAL', 'INSTITUTION'))
);

CREATE TABLE CREDIT_TERM
(
    CREDITSTATUS VARCHAR(20) NOT NULL,
    STARTDATE DATE NOT NULL,
    ENDDATE DATE NOT NULL,

    CONSTRAINT CREDIT_TERM_PK PRIMARY KEY(CREDITSTATUS)
);


insert into CREDIT_TERM values('ONE-MONTH','15-05-2015','15-06-2015');
insert into CREDIT_TERM values('TWO-MONTH','15-05-2015','15-06-2015');
insert into CREDIT_TERM values('THREE-MONTH','15-05-2015','15-06-2015');

ALTER TABLE CUSTOMER 
ADD CONSTRAINT CUSTOMER_FK_CREDITSTATUS 
    FOREIGN KEY(CREDITSTATUS) REFERENCES CREDIT_TERM(CREDITSTATUS);
</code></pre>

<p>I am trying to add a foreign key constraint, but I don't understand why I get this error:</p>

<blockquote>
  <p>ERROR at last line :<br>
  ORA-00904: "CREDITSTATUS": invalid identifier</p>
</blockquote>