An aerosol-generating system is provided, including a cartridge and an aerosol-generating device; the cartridge including a cartridge body defining a substrate compartment and a heater cavity, and an aerosol-forming substrate positioned within the substrate compartment; the aerosol-generating device including a housing defining a device cavity configured to receive at least a portion of the cartridge body, and an elongate electric heater disposed in the device cavity; and a thermally conductive sheath secured over the elongate electric heater such that the electric heater is substantially enclosed within the thermally conductive sheath along at least part of a length thereof; in which the thermally conductive sheath and the electric heater are configured to extend into the heater cavity of the cartridge when the cartridge is received in the device cavity such that the substrate compartment of the cartridge is heated by the electric heater via the thermally conductive sheath.