Power spectrum of residual surface-brightness fluctuations remaining in the lensed images of SDSS J0252+0039 after the lens-galaxy subtraction, the smooth lens modelling and the noise correction. The difference between the power spectrum of residual surface-brightness fluctuations after the smooth lens modelling with n = 3 (blue line) and the estimated total noise power spectrum (green line) constitutes our upper limit on the power spectrum of surface-brightness anomalies due to mass structure in the lens galaxy SDSS J0252+0039 (red line).