Error con INSERT INTO

[alonsoandres]

tengo este codigo

Código MySQL:

INSERT INTO users (username, password, email, credits, group) 
VALUES ('$username', '$password', '$email', '0', '$group_id');
 

y me da este error

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'group) VALUES ('test', 'test', 'test', '0', '2')' at line 1

Que puede ser?