AttributeError: 'float' object has no attribute 'split' : in Printing Grannt Representation of Project and task

In Project Management if the project and task have 'Working Time' defined the Report "Grantt Representation" is causing this error:

Traceback (most recent call last):
  File "/home/tiny/workspace/new_bzr/server/bin/netsvc.py", line 284, in _dispatch
    r = m(*params)
  File "/home/tiny/workspace/new_bzr/server/bin/service/web_services.py", line 515, in report_get
    return self._check_report(report_id)
  File "/home/tiny/workspace/new_bzr/server/bin/service/web_services.py", line 491, in _check_report
    raise result['exception']
AttributeError: 'float' object has no attribute 'split'

But if we don't have anything in the many2one field 'Working Time' then the "Grantt Representation" Report works fine,