diff --git a/rubin_sim/maf/metrics/crowding_metric.py b/rubin_sim/maf/metrics/crowding_metric.py
index d76a494c..2bd6bd49 100644
--- a/rubin_sim/maf/metrics/crowding_metric.py
+++ b/rubin_sim/maf/metrics/crowding_metric.py
@@ -36,6 +36,10 @@ def _comp_crowd_error(mag_vector, lum_func, seeing, single_mag=None):
     mag_uncert : `np.array` (N,)
         Magnitude uncertainties.
     """
+    # Our saved luminosity functions are cumulative,
+    # so we must here convert to differential
+    lum_func = np.diff(lum_func)
+    mag_vector = mag_vector[1:]
     lum_area_arcsec = 3600.0**2
     lum_vector = 10 ** (-0.4 * mag_vector)
     coeff = np.sqrt(np.pi / lum_area_arcsec) * seeing / 2.0