sqlerr 1062键'abcd'的重复条目'x-y'，它是两个int列的唯一键

Question

I created a table in mysql(5.1.55) as following:

CREATE TABLE IF NOT EXISTS xyz (
  intC1 int(11) NOT NULL AUTO_INCREMENT,
  intC2 int(11) NOT NULL,
  intC3 int(11) NOT NULL,
  PRIMARY KEY (intC1),
  UNIQUE KEY abcd (intC2,intC3)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

And then I tried to insert a row (using php 5.3.6, mysql 5.1.55) as following:

insert into xyz (intC1, intC2, intC3) values (null,4, 14);

And I received the error 1062 as following:

"Duplicate entry '4-14' for key 'abcd'"

This error repeats whether there are any rows in the table or not.

Please help.