A connector assembly includes a plug connector having a base body and at least one inserting portion extended from a front of the base body. The inserting portion has a substantially circular plug groove of which an inner wall has a portion formed with an anti-mismating portion of plane shape. The anti-mismating portion has a null terminal recess for receiving a null terminal. A socket connector coupled with the plug connector defines an insulating housing having at least one receiving groove at a front surface thereof and a substantially circular receiving portion surrounded by the receiving groove. The receiving portion has a portion formed with an anti-mismating face of plane shape, corresponding to the anti-mismating portion, with a null pin recess formed thereon. A null pin is received in the null pin recess for electrically connecting with the null terminal of the plug connector.