diff --git a/common/AzimuthalIntegrationProfile.cpp b/common/AzimuthalIntegrationProfile.cpp
index 165bfe27..3b271293 100644
--- a/common/AzimuthalIntegrationProfile.cpp
+++ b/common/AzimuthalIntegrationProfile.cpp
@@ -18,7 +18,10 @@ inline float calc_std(float sum, float sum2, uint64_t count) {
         return NAN;
     const auto fp_count = static_cast<float>(count);
     const float variance = (sum2 - sum * sum / fp_count) / (fp_count - 1);
-    return std::sqrt(variance);
+    // The two sums are floats accumulated over millions of pixels, so on a near-constant ring the
+    // difference of two nearly equal large numbers lands either side of zero; clamp before the root
+    // rather than emit a NaN standard deviation. Both spot-finder ring accumulators do the same.
+    return std::sqrt(std::max(0.0f, variance));
 }
 
 AzimuthalIntegrationProfile::AzimuthalIntegrationProfile(const AzimuthalIntegrationMapping &mapping)