GraphML parsing of boolean fields problem

I've got a problem with parsing boolean fields in .graphml files.

I have the following:

...
<key attr.name="direct" attr.type="boolean" for="edge" id="d14" />
...
<data key="d14">true</data>
...

According to http://www.w3.org/TR/xmlschema-2/#boolean correct values are:

• true | True | 1
• false | False | 0

But graph-tool raises following:

IOError: error reading from file 'graph.graphml':parse error: on line 117, column 26: invalid value "true" for key direct of type boolean

Code is

g = gt.load_graph('graph.graphml', fmt='xml')

graph-tool is of version 2.2.31