diff --git a/src/surface/algo/image2mesh.h b/src/surface/algo/image2mesh.h
index f9ef180cd0..7415f43dd1 100644
--- a/src/surface/algo/image2mesh.h
+++ b/src/surface/algo/image2mesh.h
@@ -122,6 +122,7 @@ namespace MR
                     const auto existing = vox2vertindex.find (voxels[in_vertex]);
                     if (existing == vox2vertindex.end()) {
                       triangle_vertices[out_vertex] = vertices.size();
+                      vox2vertindex.insert(std::make_pair(voxels[in_vertex], vertices.size()));
                       Eigen::Vector3d pos_voxelspace (default_type(voxels[in_vertex][0]) - 0.5, default_type(voxels[in_vertex][1]) - 0.5, default_type(voxels[in_vertex][2]) - 0.5);
                       vertices.push_back (transform.voxel2scanner * pos_voxelspace);
                     } else {