Images of the two strong gravitational lensing systems used in this paper, where subhalo detections (marked by a white cross) have been claimed in previous works. Left: image of B1938+666 taken in the K′ band by NIRC2 on Keck. In this case, z_L = 0.881 and z_s = 2.059; the pixel and FWHM (determining the resolution) scales are 10 and 70 milli-arcseconds (hereafter, mas), respectively. Right: HST F814W observations of the lens J0946+1006. For this system, z_L = 0.222 and z_s = 0.609; the pixel size and resolution are 50 and 90 mas, respectively. See Table 1 for a list of previous analyses of these systems.