In the main text on page 5242 of Ref. [1], we have estimated the absolute conversion efficiencies of second- (SHG) and third-harmonic generation (THG) of resonantly excited magnetic split-ring resonators (SRR). The values quoted there of 2×10^-6 for SHG and 3×10^-7 for THG are incorrect by a factor corresponding to the multiplication factor of the photomultiplier tube used (1×10⁵). The correct values are 2×10^-11 for SHG and 3×10^-12 for THG, respectively. These correct values have independently been verified by using another detection scheme, namely by a silicon charge-coupled device (CCD) camera connected to a grating spectrometer. Note that this mistake does not affect any of the qualitative statements of the paper.