Tikz graph package: complete bipartite

I am wondering about the functionality of the tikz graph package and ran into the following issue.

I got this example form the TikZ & PGF Manual for Version 2.10-CVS:

tikz graph {
{a,b,c} -> [complete bipartite] {e,f,g}
};

Minor adjustments to the left hand side, the code still works.

tikz graph {
{$v_1$,$v_2$,$v_3$} -> [complete bipartite] {e,f,g}
};

After making the same adjustments to the right hand side, the code breaks.

tikz graph {
{$v_1$,$v_2$,$v_3$} -> [complete bipartite] {$r_1$,$r_2$,$r_3$}
};

Does anyone know why this would happen?