[BGL/iterator_adaptor] retrieving edge iterators from a const graph object?

I'm developing an algorithm based on a DFS of a directed graph that models the BidirectionalGraph concept. In my DFS visitor I have a need to retrieve the pair of in_edge iterators of a specific vertex in my finish_vertex method. Invoking boost::in_edges(Vertex, Graph) and discarding the result compiles fine. However, I can't compile the statement boost::tie(edgeiniterBegin, edgeiniterEnd) = boost::in_edges(Vertex, Graph). I'm struggling to understand why the const-ness of the Graph object results in a "no user defined conversion. from const boost::iterator_adaptor ...." compilation error. I suspect that I have incorrectly specified the type of the in-edge iterators to which I'm attempting to boost::tie the returned iterator pair (my target iterators are declared as boost::graph_traits::in_edge_iterator currently) but haven't been able to figure out how else to explain my intent to the compiler. I'll post the specific error message produced by the Windows version of the Intel 7.x compiler and source if necessary - but I'm guessing that I'm missing something pretty obvious here. Any clarification/suggestions would be appreciated. TIA - Chris