diff --git a/image_analysis/scale_merge/RotationScaleMergeGPU.cu b/image_analysis/scale_merge/RotationScaleMergeGPU.cu
index c34d9e68..9b8123f4 100644
--- a/image_analysis/scale_merge/RotationScaleMergeGPU.cu
+++ b/image_analysis/scale_merge/RotationScaleMergeGPU.cu
@@ -354,7 +354,9 @@ namespace {
                     const double corr = p.corr[i];
                     const double I_corr = pooled_I(i) * corr;
                     const double sigma_corr = double(p.sigma[i]) * corr;
-                    const double bkg_var = sigma_corr * sigma_corr - corr * I_corr;
+                    // max(0, I): a down-fluctuated partial carries no Poisson signal to remove, and
+                    // removing a negative one inflates the background part instead of leaving it alone.
+                    const double bkg_var = sigma_corr * sigma_corr - corr * Dmax(0.0, I_corr);
                     double var = Dmax(0.0, bkg_var) + corr * Dmax(0.0, F);
                     if (!(var > 0.0)) var = sigma_corr * sigma_corr;
                     const double w = 1.0 / var;