Simple projection of bipartite (two-mode) graph not working in igraph 0.7.1

Consider this simple, bipartite graph in GraphML:

<?xml version="1.0" encoding="UTF-8"?>
  <graphml xmlns="http://ift.tt/QM36aN"
       xmlns:xsi="http://ift.tt/ra1lAU"
       xsi:schemaLocation="http://ift.tt/QM36aN
         http://ift.tt/1guo4q0">
  <key id="type" for="node" attr.name="type" attr.type="boolean">
    <default>TRUE</default>
  </key>
  <graph id="G" edgedefault="directed">
    <node id='p1'><data key='type'>FALSE</data></node>
    <node id='o1'></node>
    <node id='o2'></node>
    <edge id='e1' source='p1' target='o1'></edge>
    <edge id='e2' source='p1' target='o2'></edge>
  </graph>
</graphml>

Now consider this R session:

require("igraph")
graph <- read.graph(file="bipartitetest.graphml",format="graphml")
proj <- bipartite.projection(graph)

Although the graph object seems fine:

graph
IGRAPH D--B 3 2 -- 
+ attr: type (v/l), id (v/c), id (e/c)

my igraph version 0.7.1 complains:

Error in .Call("R_igraph_bipartite_projection", graph, types, as.integer(probe1),  : 
  At bipartite.c:198 : Non-bipartite edge found in bipartite projection, Invalid value

Why is that? The simple graph seems to be valid a-priori.