diff --git a/masque/pattern.py b/masque/pattern.py
index a630552..c649543 100644
--- a/masque/pattern.py
+++ b/masque/pattern.py
@@ -1829,6 +1829,8 @@ def map_layers(
     new_elements: defaultdict[layer_t, list[TT]] = defaultdict(list)
     for old_layer, seq in elements.items():
         new_layer = map_layer(old_layer)
+        if new_layer is None:
+            raise PatternError(f'Layer mapping returned None for source layer {old_layer!r}')
         new_elements[new_layer].extend(seq)
     return new_elements
 
diff --git a/masque/test/test_library.py b/masque/test/test_library.py
index 32e7c3f..c205ec1 100644
--- a/masque/test/test_library.py
+++ b/masque/test/test_library.py
@@ -673,6 +673,15 @@ def test_layer_mapped_view_detaches_and_maps_shapes_and_labels() -> None:
     assert not hasattr(masque.library, 'PortsLibraryView')
 
 
+def test_layer_mapped_view_preflight_rejects_none_layer() -> None:
+    pattern = Pattern()
+    pattern.polygon((1, 0), vertices=[[0, 0], [1, 0], [0, 1]])
+    mapped = LayerMappedView(Library({'top': pattern}), lambda _layer: None)  # type: ignore[arg-type]
+
+    with pytest.raises(PatternError, match=r"returned None for source layer \(1, 0\)"):
+        preflight_source_aware(mapped)
+
+
 def test_layer_mapped_view_materialization_and_copy_through() -> None:
     pattern = Pattern()
     pattern.polygon('A', vertices=[[0, 0], [1, 0], [0, 1]])