diff --git a/image_analysis/scale_merge/SearchSpaceGroup.cpp b/image_analysis/scale_merge/SearchSpaceGroup.cpp
index 69a3d661..0c0d1fd0 100644
--- a/image_analysis/scale_merge/SearchSpaceGroup.cpp
+++ b/image_analysis/scale_merge/SearchSpaceGroup.cpp
@@ -863,8 +863,13 @@ SearchSpaceGroupResult SearchSpaceGroup(
             present_n > 0 ? static_cast<double>(present_strong) / present_n : 0.0;
         const double centering_violation_rate =
             centering_absent > 0 ? static_cast<double>(centering_violations) / centering_absent : 0.0;
+        // The mean-ratio test only means anything while the present class carries signal: with a
+        // present mean at or below zero the bound is non-positive and the comparison turns on the sign
+        // of the absent mean rather than on its size, accepting or rejecting a centering by accident.
+        // Leave that case to the rate test below, which counts violations and cannot change sign.
         const bool centering_ok = centering_absent == 0
-            || (present_n > 0 && centering_absent_mean <= opt.max_absent_present_ratio * present_mean)
+            || (present_n > 0 && present_mean > 0.0
+                && centering_absent_mean <= opt.max_absent_present_ratio * present_mean)
             || (present_strong_rate > 0.0
                 && centering_violation_rate <= opt.max_absent_present_ratio * present_strong_rate);
         const bool screw_ok = screw_absent == 0 ||