An electrical connector system includes a receptacle connector having a receptacle housing holding a plurality of receptacle signal contacts arranged in pairs carrying differential signals. The receptacle housing has a front face. The system includes a header connector coupled to the receptacle connector. The header connector includes a header housing holding a plurality of header signal contacts arranged in pairs carrying differential signals and mated with corresponding receptacle signal contacts. The header housing has a front face that opposes the front face of the receptacle housing when coupled thereto with a gap being defined between the front faces. Gap fillers are provided within the gap. The gap fillers are conductive and include deflectable spring fingers. The gap fillers provide impedance control for the header signal contacts along the gap.