Why do I get an error "using temporary as lvalue"

I cannot understand why I am getting an "using temporary as lvalue" error.

void setEdgeLengths(const Koala::AssocArray <koalaGraph::PEdge,      Koala::DijkstraHeap::EdgeLabs<int >> &edgeMap, const     std::vector<koalaGraph::PEdge>& E) 
{
   int edgeLength = 1;
   for (const auto& e : E) {

   edgeMap[e].length = edgeLength;
}

}

The variable edgeLength is not temporary.