diff --git a/src/NwcIO/Model/Clash/NwcClashDetector.cs b/src/NwcIO/Model/Clash/NwcClashDetector.cs
index 5b31f72..7e61034 100644
--- a/src/NwcIO/Model/Clash/NwcClashDetector.cs
+++ b/src/NwcIO/Model/Clash/NwcClashDetector.cs
@@ -12,10 +12,12 @@ public static class NwcClashDetector
     /// <summary>
     /// Executes clash detection across an individual Navisworks document according to the specified test configuration.
     /// </summary>
+    /// <exception cref="ArgumentException">A selection names a set that the document doesn't contain.</exception>
     public static NwcClashTest Run(NwcDocument document, NwcClashTest test)
     {
         ArgumentNullException.ThrowIfNull(document);
         ArgumentNullException.ThrowIfNull(test);
+        ThrowIfSetIsMissing([document], test);
 
         var meshesA = ResolveSelection(document, test.SelectionA);
         var meshesB = ResolveSelection(document, test.SelectionB);
@@ -27,10 +29,12 @@ public static NwcClashTest Run(NwcDocument document, NwcClashTest test)
     /// <summary>
     /// Executes clash detection across all contributing models of a federated Navisworks assembly.
     /// </summary>
+    /// <exception cref="ArgumentException">A selection names a set that none of the models contains.</exception>
     public static NwcClashTest Run(NwcAssembly assembly, NwcClashTest test)
     {
         ArgumentNullException.ThrowIfNull(assembly);
         ArgumentNullException.ThrowIfNull(test);
+        ThrowIfSetIsMissing(assembly.Models, test);
 
         var itemsA = new List<(NwcMesh Mesh, string ModelName, string? Layer)>();
         var itemsB = new List<(NwcMesh Mesh, string ModelName, string? Layer)>();
@@ -350,9 +354,29 @@ private static IReadOnlyList<NwcMesh> ResolveSelection(NwcDocument document, Nwc
                     matchedMeshes.AddRange(document.Meshes.Where(m => m.Node is not null && nodeSet.Contains(m.Node)));
                 }
             }
-            if (matchedMeshes.Count > 0) return matchedMeshes;
+            // Only the named sets, even when they match nothing: a model without them adds no meshes
+            return matchedMeshes;
         }
 
         return document.Meshes;
     }
+
+    /// <summary>
+    /// Throws when a set named by a selection is in none of the models, so a misspelled name can't run against
+    /// nothing without notice. A set that exists in only some models of an assembly is fine.
+    /// </summary>
+    private static void ThrowIfSetIsMissing(IReadOnlyList<NwcDocument> documents, NwcClashTest test)
+    {
+        foreach (var selection in new[] { test.SelectionA, test.SelectionB })
+        {
+            foreach (string name in selection.SetNames)
+            {
+                if (!documents.Any(d => d.FindSet(name) is not null))
+                {
+                    throw new ArgumentException(
+                        $"Clash test '{test.Name}' selects the set '{name}', which no model contains.", nameof(test));
+                }
+            }
+        }
+    }
 }
diff --git a/tests/NwcIO.Tests/ClashDetectiveTests.cs b/tests/NwcIO.Tests/ClashDetectiveTests.cs
index 762407d..b69ede0 100644
--- a/tests/NwcIO.Tests/ClashDetectiveTests.cs
+++ b/tests/NwcIO.Tests/ClashDetectiveTests.cs
@@ -104,6 +104,78 @@ public void AssemblyClashDetection_ReportsOnlyUniqueCrossModelPairs(bool interse
         Assert.All(test.Results, clash => Assert.NotEqual(clash.Item1.ModelName, clash.Item2.ModelName));
     }
 
+    private static NwcDocument CreateClashingDocument(string name, params Model.Sets.NwcSetItem[] sets)
+    {
+        var builder = new NwcDocumentBuilder().SetFileInfo(name)
+            .AddMesh(CreateBoxMesh(1, new(0, 0, 0), new(2, 2, 2)))
+            .AddMesh(CreateBoxMesh(2, new(1, 0, 0), new(3, 2, 2)));
+        foreach (var set in sets)
+        {
+            builder.AddSelectionSet(set);
+        }
+
+        return builder.Build();
+    }
+
+    [Fact]
+    public void SetSelection_UnknownSetName_Throws()
+    {
+        var doc = CreateClashingDocument("Model.nwc");
+        var test = new NwcClashTest("Typo", NwcClashTestType.Hard, tolerance: 0.01)
+        {
+            SelectionA = NwcClashSelection.FromSet("NoSuchSet"),
+            SelectionB = NwcClashSelection.All(),
+        };
+
+        var ex = Assert.Throws<ArgumentException>(() => NwcClashDetector.Run(doc, test));
+        Assert.Contains("NoSuchSet", ex.Message, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SetSelection_SetWithoutMeshes_SelectsNothing()
+    {
+        var doc = CreateClashingDocument("Model.nwc", new Model.Sets.NwcSelectionSet("Empty", [999]));
+        var test = new NwcClashTest("Empty set", NwcClashTestType.Hard, tolerance: 0.01)
+        {
+            SelectionA = NwcClashSelection.FromSet("Empty"),
+            SelectionB = NwcClashSelection.All(),
+        };
+
+        NwcClashDetector.Run(doc, test);
+
+        Assert.Equal(0, test.TotalClashCount);
+    }
+
+    [Fact]
+    public void SetSelection_SetInOneModelOnly_SelectsNothingFromTheOthers()
+    {
+        var architecture = CreateClashingDocument("Architecture.nwc", new Model.Sets.NwcSelectionSet("Empty", [999]));
+        var mep = CreateClashingDocument("MEP.nwc");
+        var test = new NwcClashTest("Federated", NwcClashTestType.Hard, tolerance: 0.01)
+        {
+            SelectionA = NwcClashSelection.FromSet("Empty"),
+            SelectionB = NwcClashSelection.All(),
+        };
+
+        NwcClashDetector.Run(NwcAssembly.Create("Federated", architecture, mep), test);
+
+        Assert.Equal(0, test.TotalClashCount);
+    }
+
+    [Fact]
+    public void SetSelection_SetMissingFromEveryModel_Throws()
+    {
+        var assembly = NwcAssembly.Create("Federated", CreateClashingDocument("A.nwc"), CreateClashingDocument("B.nwc"));
+        var test = new NwcClashTest("Typo", NwcClashTestType.Hard, tolerance: 0.01)
+        {
+            SelectionA = NwcClashSelection.All(),
+            SelectionB = NwcClashSelection.FromSet("NoSuchSet"),
+        };
+
+        var ex = Assert.Throws<ArgumentException>(() => NwcClashDetector.Run(assembly, test));
+        Assert.Contains("NoSuchSet", ex.Message, StringComparison.Ordinal);
+    }
+
     [Fact]
     public void IgnoreSameLayerRuleFiltersOutSameLayerClashes()
     {