diff --git a/source/blender/blenkernel/BKE_mesh_legacy_convert.h b/source/blender/blenkernel/BKE_mesh_legacy_convert.h
index 8a0ae63ce72..10c450cb6ec 100644
--- a/source/blender/blenkernel/BKE_mesh_legacy_convert.h
+++ b/source/blender/blenkernel/BKE_mesh_legacy_convert.h
@@ -29,7 +29,8 @@ struct MFace;
 void BKE_mesh_legacy_convert_uvs_to_struct(
     Mesh *mesh,
     blender::ResourceScope &temp_mloopuv_for_convert,
-    blender::Vector<CustomDataLayer, 16> &loop_layers_to_write);
+    blender::Vector<CustomDataLayer, 16> &loop_layers_to_write,
+    const int mesh_ldata_totlayer_real);
 void BKE_mesh_legacy_convert_uvs_to_generic(Mesh *mesh);
 
 /**
@@ -121,6 +122,8 @@ struct MLoop *BKE_mesh_legacy_convert_corners_to_loops(
 
 void BKE_mesh_legacy_convert_loops_to_corners(struct Mesh *mesh);
 
+void BKE_mesh_assert_customdata_alloc(Mesh *mesh);
+
 /**
  * Recreate #MFace Tessellation.
  *
diff --git a/source/blender/blenkernel/intern/mesh.cc b/source/blender/blenkernel/intern/mesh.cc
index cf71ecf0270..29acb05969a 100644
--- a/source/blender/blenkernel/intern/mesh.cc
+++ b/source/blender/blenkernel/intern/mesh.cc
@@ -253,6 +253,8 @@ static void mesh_blend_write(BlendWriter *writer, ID *id, const void *id_address
   }
   else {
     Set<std::string> names_to_skip;
+
+    const int mesh_ldata_totlayer_real = mesh->ldata.totlayer;
     if (!BLO_write_is_undo(writer)) {
       /* When converting to the old mesh format, don't save redundant attributes. */
       names_to_skip.add_multiple_new({"position",
@@ -296,7 +298,11 @@ static void mesh_blend_write(BlendWriter *writer, ID *id, const void *id_address
     CustomData_blend_write_prepare(mesh->pdata, poly_layers, names_to_skip);
 
     if (!BLO_write_is_undo(writer)) {
-      BKE_mesh_legacy_convert_uvs_to_struct(mesh, temp_arrays_for_legacy_format, loop_layers);
+      /* If ASAN isn't doing it's job, assert! */
+      // BKE_mesh_assert_customdata_alloc(mesh);
+
+      BKE_mesh_legacy_convert_uvs_to_struct(
+          mesh, temp_arrays_for_legacy_format, loop_layers, mesh_ldata_totlayer_real);
       BKE_mesh_legacy_face_set_from_generic(poly_layers);
     }
   }
diff --git a/source/blender/blenkernel/intern/mesh_legacy_convert.cc b/source/blender/blenkernel/intern/mesh_legacy_convert.cc
index b6977cd9855..63f5741bf37 100644
--- a/source/blender/blenkernel/intern/mesh_legacy_convert.cc
+++ b/source/blender/blenkernel/intern/mesh_legacy_convert.cc
@@ -1658,7 +1658,8 @@ void BKE_mesh_legacy_convert_mpoly_to_material_indices(Mesh *mesh)
 void BKE_mesh_legacy_convert_uvs_to_struct(
     Mesh *mesh,
     blender::ResourceScope &temp_mloopuv_for_convert,
-    blender::Vector<CustomDataLayer, 16> &loop_layers_to_write)
+    blender::Vector<CustomDataLayer, 16> &loop_layers_to_write,
+    const int mesh_ldata_totlayer_real)
 {
   using namespace blender;
   using namespace blender::bke;
@@ -1693,6 +1694,10 @@ void BKE_mesh_legacy_convert_uvs_to_struct(
         mesh->totloop);
     mloopuv_layer.data = mloopuv.data();
 
+    /* FIXME: proof of concept, works but should not be necessary. */
+    const int mesh_ldata_totlayer_for_write = mesh->ldata.totlayer;
+    mesh->ldata.totlayer = mesh_ldata_totlayer_real;
+
     char buffer[MAX_CUSTOMDATA_LAYER_NAME];
     const bool *vert_selection = static_cast<const bool *>(CustomData_get_layer_named(
         &mesh->ldata, CD_PROP_BOOL, BKE_uv_map_vert_select_name_get(layer.name, buffer)));
@@ -1701,6 +1706,8 @@ void BKE_mesh_legacy_convert_uvs_to_struct(
     const bool *pin = static_cast<const bool *>(CustomData_get_layer_named(
         &mesh->ldata, CD_PROP_BOOL, BKE_uv_map_pin_name_get(layer.name, buffer)));
 
+    mesh->ldata.totlayer = mesh_ldata_totlayer_for_write;
+
     threading::parallel_for(mloopuv.index_range(), 2048, [&](IndexRange range) {
       for (const int i : range) {
         copy_v2_v2(mloopuv[i].uv, coords[i]);
@@ -2183,3 +2190,12 @@ void BKE_mesh_legacy_convert_loops_to_corners(Mesh *mesh)
 }
 
 /** \} */
+
+void BKE_mesh_assert_customdata_alloc(Mesh *mesh)
+{
+  CustomData *cdata[] = {&mesh->vdata, &mesh->edata, &mesh->ldata, &mesh->pdata};
+  for (size_t i = 0; i < ARRAY_SIZE(cdata); i++) {
+    BLI_assert(int(MEM_allocN_len(cdata[i]->layers) / sizeof(*cdata[i]->layers)) >=
+               cdata[i]->totlayer);
+  }
+}