Comment 1 for bug 1168894

small test case:

f <- data.frame(beg=c("a","a","b","b","c","c"),end=c("1","2","1","2","1","2"),weight=1:6)
g <- graph.data.frame(f,vertices=data.frame(vertex=c("a","b","c","1","2"),type=c("b","b","b","e","e")))
bipartite.projection(g,probe1="a")

Error in bipartite.projection(g, probe1 = "a") :
  negative length vectors are not allowed

 *** caught segfault ***
address 0x4c, cause 'memory not mapped'

Traceback:
 1: .Call("R_igraph_finalizer", PACKAGE = "igraph")
 2: bipartite.projection(g, probe1 = "a")