diff --git a/image_analysis/scale_merge/RotationScaleMerge.cpp b/image_analysis/scale_merge/RotationScaleMerge.cpp
index 87b44d2e0..117f120a9 100644
--- a/image_analysis/scale_merge/RotationScaleMerge.cpp
+++ b/image_analysis/scale_merge/RotationScaleMerge.cpp
@@ -7592,7 +7592,9 @@ RotationScaleMerge::Result RotationScaleMerge::RunArm(bool scale_partials, bool
         BuildFullsGroupCSR(n_groups);
 
     // --- 4. Scale the fulls (XDS order, Unity model). ---
-    if (scale_fulls && !scaled_fulls_on_gpu) {
+    // On the fulls the host combined: the per-frame CSR this loop slices is built by Combine(). Fulls
+    // combined on the GPU were scaled there - unless there were none, and then there is nothing to scale.
+    if (scale_fulls && !combined_on_gpu) {
         // On a working copy of the fulls, as the partials' loop (ScalingRef and ScalingFit in the header):
         // full_index[q] is the full at position q, in the group CSR's order and then the fulls in no group.
         const int nf = static_cast<int>(fulls.size());
diff --git a/tests/RotationScaleMergeSegmentsTest.cpp b/tests/RotationScaleMergeSegmentsTest.cpp
index f20d05f49..92fa4325e 100644
--- a/tests/RotationScaleMergeSegmentsTest.cpp
+++ b/tests/RotationScaleMergeSegmentsTest.cpp
@@ -229,3 +229,18 @@ TEST_CASE("RotationScaleMergeSegments_NoSegmentsIsTheEngineAsBefore", "[Rotation
     else if (get_gpu_description().find("RTX 5080") != std::string::npos)
         CHECK(hash == 6594155396455850264ULL);
 }
+
+// A sweep in which no rocking event is captured whole enough to make a full - as in one exposed 0.2 deg out
+// of every 0.6 deg, where each reflection is caught on one frame and only in part. On the GPU the combine
+// returned no fulls, the scale-fulls step then fell through to the host loop, which slices a per-frame CSR
+// that only the host combine builds, and the process died on SIGSEGV instead of reporting the empty merge.
+TEST_CASE("RotationScaleMerge_NoFullsIsAnErrorNotACrash", "[RotationScale]") {
+    const std::vector<SweepSpec> sweeps{{60, 1.0, 0.0}};
+    auto data = MakeSweeps(sweeps, 17);
+    auto x = SyntheticExperiment(false);
+    // More than any rocking event can capture: the combine makes no full.
+    ScalingSettings s = x.GetScalingSettings();
+    s.MinCapturedFraction(1000.0);
+    x.ImportScalingSettings(s);
+    CHECK_THROWS_AS(Merge(data, x, {}), JFJochException);
+}