A header assembly includes a header housing having a header cavity. The header assembly includes header signal contacts received in corresponding signal contact channels having mating ends arranged in the header cavity for mating with the receptacle assembly. The header assembly includes header ground contacts received in corresponding ground contact channels. Each header ground contact includes shield walls forming a shield cavity receiving header signal contacts to provide electrical shielding for the header signal contacts. The shield walls include an end wall extending between first and second side walls. Each header ground contact includes a mating protrusion that extends outward relative to the shield cavity from the corresponding shield wall. The mating protrusion is configured to engage a conductive insert of the receptacle assembly used to electrically common each of the header ground contacts.