[igraph] bipartite.projection() with non numeric vertices id

[capitano . nemo]

I have a bipartite graph of an online forum. The vertices id are the 
user id (e.g. "24242434") and url links (e.g. 
"www.somesite.com/somethread.html). I successfully create the graph with

graph.data.frame(edges, directed=TRUE, vertices=nodes)

IGRAPH DN-B 154612 448387 --
+ attr: name (v/c), gender (v/c), member (v/l), timestamp (v/c), type
  (v/l), reactions (v/n), dislikes (v/n), createdAt (v/c), slug (v/c),
  postNumber (v/n), likes (v/n), message (v/c), score (v/n),
  categoryLink (v/c), createdAt (e/c), type (e/c), parent (e/c),
  dislikes (e/n), message (e/c), numReports (e/n), likes (e/n)

Now I want a one-mode projection of the graph. But when I try

> igraph_onemode <- bipartite.projection(igraph_bi)

I get

Error in bipartite.projection(igraph_bi) :
  At structure_generators.c:84 : Invalid (negative) vertex id, Invalid 
vertex id

This error popped up few times when googling it. Still the discussions I 
found (I guess) don't apply to my problem since I can't remove the 
negative ids.

Thanks