diff --git a/src/libslic3r/PrintObject.cpp b/src/libslic3r/PrintObject.cpp
index 526b7b874a..808bdd8a1b 100644
--- a/src/libslic3r/PrintObject.cpp
+++ b/src/libslic3r/PrintObject.cpp
@@ -2301,7 +2301,14 @@ void PrintObject::project_and_append_custom_facets(
                 else {
                     std::vector<Polygons> projected;
                     slice_mesh_slabs(custom_facets, zs_from_layers(this->layers()), this->trafo_centered() * mv->get_matrix(), nullptr, &projected, [](){});
-                    append(out, std::move(projected));
+                    // Merge these projections with the output, layer by layer.
+                    assert(! projected.empty());
+                    assert(out.empty() || out.size() == projected.size());
+                    if (out.empty())
+                        out = std::move(projected);
+                    else
+                        for (size_t i = 0; i < out.size(); ++ i)
+                            append(out[i], std::move(projected[i]));
                 }
         }
 }