diff --git a/rugnux/RugnuxPreScan.cpp b/rugnux/RugnuxPreScan.cpp
index 5061f0d2e..b2131b876 100644
--- a/rugnux/RugnuxPreScan.cpp
+++ b/rugnux/RugnuxPreScan.cpp
@@ -190,10 +190,12 @@ void Rugnux::PreScan(int start_image, int images_to_process, int frame_count, Ru
     // radius the first one measured rather than re-reading the frames.
     const bool want_width = config_.adaptive_integration_radius && !spot_width_measured_;
     // Defective pixels, for the same reason on the same pass: each pixel is compared against its
-    // resolution ring. Only under rotation, where a reflection moves off a pixel and a defect does not.
+    // resolution ring. Only under rotation, where a reflection moves off a pixel and a defect does not -
+    // except for the pixels holding the detector's error value and the dead ones, which need no
+    // rotation and are measured on stills too (MaskDefectivePixels).
     const auto goniometer = experiment_.GetGoniometer();
     const bool want_hot = !hot_pixels_measured_
-                          && experiment_.IsRotationIndexing() && goniometer && goniometer->IsScanning();
+                          && (!experiment_.IsRotationIndexing() || (goniometer && goniometer->IsScanning()));
     // The background noise per unit level of a CCD, read off the same frames (BackgroundGain.h).
     const bool want_gain = config_.counts_in_adu && !background_gain_measured_;
     if (!want_shadow && !want_width && !want_hot && !want_gain)
@@ -1193,9 +1195,11 @@ void Rugnux::MaskDefectivePixels(int start_image, const std::vector<int> &sample
         }
     });
 
+    // On stills there is no rotation to count a reflection's stay on a pixel in, so spacing stays 0 and
+    // the hot-pixel rule is not applied (HotPixelFinder::GetMask); the error-value and dead pixels are.
     const auto goniometer = experiment_.GetGoniometer();
-    const double oscillation = std::abs(goniometer->GetIncrement_deg());
-    const double spacing = sample.size() > 1
+    const double oscillation = goniometer ? std::abs(goniometer->GetIncrement_deg()) : 0.0;
+    const double spacing = goniometer && sample.size() > 1
                            ? oscillation * (sample.back() - sample.front()) / static_cast<double>(sample.size() - 1)
                            : 0.0;
     const auto hot = finder.GetMask(oscillation, spacing, nthreads);