diff --git a/image_analysis/scale_merge/RotationScaleMerge.cpp b/image_analysis/scale_merge/RotationScaleMerge.cpp
index 81291b08..030adda7 100644
--- a/image_analysis/scale_merge/RotationScaleMerge.cpp
+++ b/image_analysis/scale_merge/RotationScaleMerge.cpp
@@ -1599,13 +1599,17 @@ RotationScaleMerge::Result RotationScaleMerge::MergeAndStats(int n_groups, bool
     }
     // Guard a degenerate low-multiplicity fit: with too few symmetry equivalents both the (a, b) fit and
     // the per-group scatter collapse toward zero, and 1/b then reports an impossibly high asymptotic
-    // I/sigma. Real macromolecular data does not exceed ISa ~50; past a generous cap treat the asymptote
-    // as unmeasured (result.isa left undetermined) rather than emit a spurious extreme.
+    // I/sigma. Real macromolecular data does not exceed ISa ~50; past a generous cap report the asymptote
+    // as unmeasured rather than emit a spurious extreme. Only the REPORT is dropped: the sigma floor
+    // below keeps using the fitted value, because a degenerate fit is precisely where the raw
+    // 1/sqrt(sum_w) is least trustworthy and leaving the merged sigma uncapped would be the opposite of
+    // what this guard is for.
     constexpr double MIN_ASYMPTOTIC_B = 0.01;   // ISa cap 100
-    if (error_model_b_asymptotic < MIN_ASYMPTOTIC_B) error_model_b_asymptotic = 0.0;
+    const double isa_reported = error_model_b_asymptotic >= MIN_ASYMPTOTIC_B
+                                ? 1.0 / error_model_b_asymptotic : 0.0;
     if (error_model_active)
         logger.Info("Error model: a={:.3f} b={:.3f} ISa={:.1f} chi2={:.2f}", error_model_a, error_model_b,
-                    error_model_b_asymptotic > 0 ? 1.0 / error_model_b_asymptotic : 0.0, error_model_chi2);
+                    isa_reported, error_model_chi2);
 
     auto corrected_sigma = [&](float I_corr, float sigma_corr, int g) -> float {
         if (!error_model_active) return sigma_corr;
@@ -1662,7 +1666,7 @@ RotationScaleMerge::Result RotationScaleMerge::MergeAndStats(int n_groups, bool
         }
     // ---- Export merged reflections (+ resolution-shell R-free flags). ----
     Result result;
-    result.isa = error_model_b_asymptotic > 0 ? 1.0 / error_model_b_asymptotic : 0.0;
+    result.isa = isa_reported;
     std::vector<double> merged_I(n_groups, NAN);
     float d_min = std::numeric_limits<float>::max(), d_max = 0.0f;
     for (int g = 0; g < n_groups; ++g) {