The invention relates to an electrical connector system comprising a plug connector and a second locking device. The plug connector comprises at least one row of terminal cavities for receiving terminals and a plug connector housing having a flexible arm arranged in a sidewall of the plug connector housing, and having primary locking means adapted to provide a primary locking function. The secondary locking device is arranged between an end of the at least one row of terminal cavities and the flexible arm, and has a flexible leg with secondary locking means for providing a secondary locking function. The flexible arm comprises a blocking device and the flexible leg comprises an abutment device adapted to engage with the blocking device for said blocking.