First commit and dulwich.diff_tree.tree_changes

I am iterating repo.get_walker() and for each entry I am getting dulwich.diff_tree.tree_changes(repo, repo[entry.commit.parents[0]].tree, entry.commit.tree). This works well when entry.commit has parents. But first commit has no parents and that's when error occurs. Same happens with write_tree_diff().

What should I use instead entry.commit.parents[0] for first commit (when commit has no parents)?