diff --git a/writer/HDF5NXmx.cpp b/writer/HDF5NXmx.cpp
index e3bb7b837..1010833a8 100644
--- a/writer/HDF5NXmx.cpp
+++ b/writer/HDF5NXmx.cpp
@@ -659,21 +659,28 @@ void NXmx::Metrology(const StartMessage &start, const EndMessage &end) {
     // the sample - so on a tilted detector it is not the beam position, and a program that wants the
     // beam position (XDS's ORGX/ORGY, for one) would otherwise have to redo the tilt arithmetic. Built
     // from the very values written just above and just below, so the three can never disagree.
-    DiffractionGeometry geometry;
-    geometry.BeamX_pxl(beam_center_x)
-            .BeamY_pxl(beam_center_y)
-            .DetectorDistance_mm(start.detector_distance * 1e3f)
-            .PixelSize_mm(start.pixel_size_x * 1e3f)
-            .Orientation(DetectorOrientation(start.detector_orientation_mirror_y,
-                                             start.detector_orientation_quarter_turns))
-            .PoniRot1_rad(static_cast<float>(rot1))
-            .PoniRot2_rad(static_cast<float>(rot2))
-            .PoniRot3_rad(static_cast<float>(rot3));
-    const auto direct_beam = geometry.GetDirectBeam_pxl();
-    SaveScalar(*hdf5_file, "/entry/instrument/detector/detectorSpecific/direct_beam_x",
-               direct_beam.first)->Units("pixel");
-    SaveScalar(*hdf5_file, "/entry/instrument/detector/detectorSpecific/direct_beam_y",
-               direct_beam.second)->Units("pixel");
+    //
+    // Only where there is a geometry to compute it from. DiffractionGeometry refuses a distance under
+    // 1 mm or a pixel size of zero, and a message can legitimately carry neither - a writer told to
+    // prove it can create the files never describes a detector. This dataset is a convenience for
+    // whoever reads the file later, so it is absent there rather than fatal.
+    if (start.detector_distance * 1e3f >= 1.0f && start.pixel_size_x > 0.0f) {
+        DiffractionGeometry geometry;
+        geometry.BeamX_pxl(beam_center_x)
+                .BeamY_pxl(beam_center_y)
+                .DetectorDistance_mm(start.detector_distance * 1e3f)
+                .PixelSize_mm(start.pixel_size_x * 1e3f)
+                .Orientation(DetectorOrientation(start.detector_orientation_mirror_y,
+                                                 start.detector_orientation_quarter_turns))
+                .PoniRot1_rad(static_cast<float>(rot1))
+                .PoniRot2_rad(static_cast<float>(rot2))
+                .PoniRot3_rad(static_cast<float>(rot3));
+        const auto direct_beam = geometry.GetDirectBeam_pxl();
+        SaveScalar(*hdf5_file, "/entry/instrument/detector/detectorSpecific/direct_beam_x",
+                   direct_beam.first)->Units("pixel");
+        SaveScalar(*hdf5_file, "/entry/instrument/detector/detectorSpecific/direct_beam_y",
+                   direct_beam.second)->Units("pixel");
+    }
 
     HDF5Group transformations(*hdf5_file, "/entry/instrument/detector/transformations");
     transformations.NXClass("NXtransformations");