Provided is a cassette unit suitable for use with an auto-injector having an electrically powered drive unit. The cassette unit has a housing defining a cassette unit housing cavity and a needle projection aperture. The housing cavity is arranged for receipt of a syringe for injected delivery of a drug to a patient. The cassette unit includes a removable cap that in a capping position fits over and thereby acts to close off the needle projection aperture; and connecting to the removable cap, a needle cover defining a needle sheath for sheathing the needle tip of the syringe. The removable cap is provided with one or more first engagement features arranged for selectively engaging one or more second engagement features of the cassette unit housing when the removable cap is in the capping position to thereby restrict/prevent rotation of the removable cap and needle cover relative to the housing.