# Problem with nodes in TikZ graphs with graphdrawing library over LuaTeX

I am trying to refer to nodes set within a graph structure deployed by using the graphdrawing library but I don't get the wished for result. This is my MWE, which is compiled over LuaLaTeX in order to have the functionality of the library available:

%\RequirePackage{luatex85} <- Not really needed, I believe

\documentclass{article}

\usepackage{tikz}

\usetikzlibrary{graphs,graphdrawing}
\usegdlibrary{trees}

\pagestyle{empty}

\begin{document}
\centering
\begin{tikzpicture}[binary tree layout]
\graph{a -- b};
\end{tikzpicture}
\end{document}


I'd like to have a circle centered on each node, but what I get instead is the two circles centered both on node "a". I may be doing something in the wrong way here, but I don't know what.

It works if you add the binary tree layout option to the \graph instead of the tikzpicture:

\documentclass{article}

\usepackage{tikz}

\usetikzlibrary{graphs,graphdrawing}
\usegdlibrary{trees,circular}

\pagestyle{empty}

\begin{document}
\begin{tikzpicture}[binary tree layout]
\graph
{a -- b};