Calibration of the unWISE (Schlafly et al. 2019) W1, W2 magnitudes to the 2MASS K magnitudes of the RR Lyrae sample of Layden et al. (2019). On the vertical axis in the upper panel, we plot dereddened magnitudes K(W1, W2) = K0 (see Eq. (4)). The reference levels are shown by continuous lines, ΔK = K(L19)−K(W1, W2). The strongest outlier in the lower panel is AR Per, with an outstandingly high reddening of E(B − V) = 1.110.