diff --git a/source/blender/blenkernel/intern/customdata.cc b/source/blender/blenkernel/intern/customdata.cc
index 284440933a3..99ee56ba63d 100644
--- a/source/blender/blenkernel/intern/customdata.cc
+++ b/source/blender/blenkernel/intern/customdata.cc
@@ -4217,6 +4217,16 @@ void CustomData_blend_write_prepare(CustomData &data,
   }
   data.totlayer = layers_to_write.size();
   data.maxlayer = data.totlayer;
+
+  /* Note: data->layers may be null, this happens when adding
+   * a legacy MPoly struct to a mesh with no other face attributes.
+   * This leaves us with no unique ID for DNA to identify the old
+   * data with when loading the file.
+   */
+  if (!data.layers && layers_to_write.size() > 0) {
+    /* We just need an address that's unique. */
+    data.layers = reinterpret_cast<CustomDataLayer *>(&data.layers);
+  }
 }
 
 int CustomData_sizeof(const eCustomDataType type)
diff --git a/source/blender/blenkernel/intern/mesh_legacy_convert.cc b/source/blender/blenkernel/intern/mesh_legacy_convert.cc
index c3e7e53c439..22b3a8d652c 100644
--- a/source/blender/blenkernel/intern/mesh_legacy_convert.cc
+++ b/source/blender/blenkernel/intern/mesh_legacy_convert.cc
@@ -1240,7 +1240,7 @@ void BKE_mesh_legacy_sharp_faces_from_flags(Mesh *mesh)
   using namespace blender;
   using namespace blender::bke;
   MutableAttributeAccessor attributes = mesh->attributes_for_write();
-  if (attributes.contains("sharp_face")) {
+  if (attributes.contains("sharp_face") || !CustomData_get_layer(&mesh->pdata, CD_MPOLY)) {
     return;
   }
   const Span<MPoly> polys(static_cast<const MPoly *>(CustomData_get_layer(&mesh->pdata, CD_MPOLY)),