diff --git a/src/bonsai/bonsai/bim/module/root/operator.py b/src/bonsai/bonsai/bim/module/root/operator.py
index 159f157d44..01f9480732 100644
--- a/src/bonsai/bonsai/bim/module/root/operator.py
+++ b/src/bonsai/bonsai/bim/module/root/operator.py
@@ -380,6 +380,14 @@ class AssignClass(bpy.types.Operator, tool.Ifc.Operator):
         # TODO: reload representation might lead to the object being replaced by object of the other type.
         # We probably should track it somehow and keep the original selection.
 
+        # Objects may have just been relinked into new collections (e.g. types
+        # into the "IfcTypeProduct" collection) above. Make sure the view
+        # layer's object bases are up to date before validating the selection
+        # below, otherwise a just-relinked (but perfectly valid) active object
+        # can be seen as "not in the current view layer" and be dropped,
+        # leaving `context.active_object` as None for the rest of the caller.
+        context.view_layer.update()
+
         # Validate selection and reapply it.
         current_selection = tool.Blender.validate_object_selection(*current_selection)
         tool.Blender.set_objects_selection(*current_selection)
diff --git a/src/bonsai/bonsai/core/material.py b/src/bonsai/bonsai/core/material.py
index 0a08f5e0bf..f641909a65 100644
--- a/src/bonsai/bonsai/core/material.py
+++ b/src/bonsai/bonsai/core/material.py
@@ -130,10 +130,23 @@ def assign_material(
 
         if can_be_usage and not material_tool.is_type_product(element):
             element_material_type = material_type + "Usage"
+            # A "...Usage" type always wraps an existing material *set*
+            # (IfcMaterialLayerSet/IfcMaterialProfileSet), never a plain
+            # IfcMaterial. `material` here is whatever single IfcMaterial is
+            # selected in the Object Materials UI, so it can't be passed
+            # through as-is or ifcopenshell.api.material.assign_material will
+            # assert (e.g. "IfcMaterial cannot be assiged as a
+            # IfcMaterialLayerSetUsage."). Passing None lets the API derive
+            # (or create) the proper set from the element's type; the
+            # selected material is added into that set below instead.
+            material_for_assignment = None
         else:
             element_material_type = material_type
+            material_for_assignment = material
 
-        ifc.run("material.assign_material", products=[element], type=element_material_type, material=material)
+        ifc.run(
+            "material.assign_material", products=[element], type=element_material_type, material=material_for_assignment
+        )
         assigned_material = material_tool.get_material(element)
         assert assigned_material  # Type checker.