A connector (100) comprises a housing (1), an inner board (3) and a number of contacts (2) supported by the housing. The housing has a base portion (11), a first (16) and second side wall (15). The base portion defines a receiving cavity (14) for receiving a mating connector along a front-to-back direction. The first (16) and second side wall (15) are parallel to each other and backwardly extending from the base portion to defining a mounting room (17). The inner board is inserted into the mounting room along a back-to-front direction. The inner board has a first flange (37) cooperated with the first side wall, and a second flange (36) cooperated with the second side walls. The first side wall has a length greater than the second wall, and the first flange has a length smaller than the second flange. This relationship is used to prevent incorrect orientation of the inner board in the housing.