diff --git a/src/Mod/Assembly/App/AssemblyUtils.cpp b/src/Mod/Assembly/App/AssemblyUtils.cpp
index 8569af29f4..8ab3c064fd 100644
--- a/src/Mod/Assembly/App/AssemblyUtils.cpp
+++ b/src/Mod/Assembly/App/AssemblyUtils.cpp
@@ -164,6 +164,49 @@ DistanceType getDistanceType(App::DocumentObject* joint)
     auto* obj1 = getLinkedObjFromRef(joint, "Reference1");
     auto* obj2 = getLinkedObjFromRef(joint, "Reference2");
 
+    // Datum planes (App::Plane) have empty element types because their
+    // sub-name ends with "." and yields no Face/Edge/Vertex element.
+    // Detect them here and classify before the main geometry chain,
+    // which cannot handle the empty element type.
+    const bool datum1 = type1.empty() && obj1 && obj1->isDerivedFrom<App::Plane>();
+    const bool datum2 = type2.empty() && obj2 && obj2->isDerivedFrom<App::Plane>();
+
+    if (datum1 || datum2) {
+        if (datum1 && datum2) {
+            return DistanceType::PlanePlane;
+        }
+
+        // One side is a datum plane, the other has a real element type.
+        // For PointPlane/LinePlane, the solver's PointInPlaneConstraint
+        // reads the plane normal from marker_j (Reference2).  Unlike
+        // real Face+Vertex joints (where both Placements carry the
+        // face normal from findPlacement), datum planes only carry
+        // their normal through computeMarkerTransform.  So the datum
+        // must end up on Reference2 for the normal to reach marker_j.
+        //
+        // For PlanePlane the convention matches the existing Face+Face
+        // path (plane on Reference1).
+        const auto& otherType = datum1 ? type2 : type1;
+
+        if (otherType == "Vertex" || otherType == "Edge") {
+            // Datum must be on Reference2 (j side).
+            if (datum1) {
+                swapJCS(joint);  // move datum from Ref1 → Ref2
+            }
+            if (otherType == "Vertex") {
+                return DistanceType::PointPlane;
+            }
+            return DistanceType::LinePlane;
+        }
+
+        // Face + datum or unknown + datum → PlanePlane
+        // Datum on Reference1 for consistency with Face+Face path.
+        if (!datum1) {
+            swapJCS(joint);  // move datum from Ref2 → Ref1
+        }
+        return DistanceType::PlanePlane;
+    }
+
     if (type1 == "Vertex" && type2 == "Vertex") {
         return DistanceType::PointPoint;
     }