An electrical connector in combination with a complementary electrical connector, the electrical connector comprising: a stack of contact housings, each accommodating one or more contacts; and a lock housing that locks the stack of contact housings in a stacked state, wherein the lock housing comprises a base portion that is stacked on one said contact housing located at a first end of the stack of contact housings; and a lock arm portion that extends from the base portion towards a further said contact housing located at an opposite second end of the stack of contact housings along lateral faces of the stacked contact housings, and engages with at least the contact housing located at the second end of the stack, and wherein the lock arm portion has an engaging portion configured to be engaged with some of the contact housings, characterised in that the engaging portion of the lock arm portion is configured to be engaged with each of the contact housings, and in a situation in which the engaging portion is in a non-engaged state with the contact housings, the engaging portion interferes with contact housings so that the lock arm portion is deformed in a direction away from the lateral faces of each contact housing, wherein a hood portion having a cylindrical shape is formed on the complementary electrical connector and is configured to receive the electrical connector, and wherein in a situation in which the lock arm portion is deformed in the direction away from the lateral faces of each of the contact housings, upon inserting the electrical connector into the hood portion of the complimentary electrical connector, the deformed lock arm portion interferes with the hood portion. a stack of contact housings, each accommodating one or more contacts; and a lock housing that locks the stack of contact housings in a stacked state, wherein the lock housing comprises a base portion that is stacked on one said contact housing located at a first end of the stack of contact housings; and a lock arm portion that extends from the base portion towards a further said contact housing located at an opposite second end of the stack of contact housings along lateral faces of the stacked contact housings, and engages with at least the contact housing located at the second end of the stack, and wherein the lock arm portion has an engaging portion configured to be engaged with some of the contact housings, characterised in that the engaging portion of the lock arm portion is configured to be engaged with each of the contact housings, and in a situation in which the engaging portion is in a non-engaged state with the contact housings, the engaging portion interferes with contact housings so that the lock arm portion is deformed in a direction away from the lateral faces of each contact housing, wherein a hood portion having a cylindrical shape is formed on the complementary electrical connector and is configured to receive the electrical connector, and wherein in a situation in which the lock arm portion is deformed in the direction away from the lateral faces of each of the contact housings, upon inserting the electrical connector into the hood portion of the complimentary electrical connector, the deformed lock arm portion interferes with the hood portion.