[igraph] Creating a bipartite graph

[Mouly]

I'm trying to create a bipartite graph (one group is a numeric ids, and the other is a string) with the following command:

g <- graph.bipartite(rep(0:1,10),c('1','a','2','c','3','a',1,'c','4','d'))

I get the error message 

Error in graph.bipartite(rep(0:1, 10), c("1", "a", "2", "c", "3", "a",  : 

  At bipartite.c:456 : Invalid edges, not a bipartite graph, Invalid value

In addition: Warning message:

In graph.bipartite(rep(0:1, 10), c("1", "a", "2", "c", "3", "a",  :

  NAs introduced by coercion

Can someone show me the correct way to create a bipartite graph? I don't understand how igraph validates that the edges do belong to different groups. 

-mouly