TypeError: _delta_to_float() takes exactly 3 arguments (2 given)

When using SST with a new testtools (1.7, 1.8), the following failures is encountered after the first test is run:

OKstopping virtual display...
Traceback (most recent call last):
  File "/home/user/example/env/bin/sst-run", line 11, in <module>
    sys.exit(main())
  File "/home/user/example/env/local/lib/python2.7/site-packages/sst/scripts/run.py", line 64, in main
    excludes=cmd_opts.excludes
  File "/home/user/example/env/local/lib/python2.7/site-packages/sst/runtests.py", line 115, in runtests
    suite.run(result)
  File "/usr/lib/python2.7/unittest/suite.py", line 108, in run
    test(result)
  File "/usr/lib/python2.7/unittest/suite.py", line 70, in __call__
    return self.run(*args, **kwds)
  File "/usr/lib/python2.7/unittest/suite.py", line 108, in run
    test(result)
  File "/home/user/example/env/local/lib/python2.7/site-packages/unittest2/case.py", line 649, in __call__
    return self.run(*args, **kwds)
  File "/home/user/example/env/local/lib/python2.7/site-packages/testtools/testcase.py", line 606, in run
    return run_test.run(result)
  File "/home/user/example/env/local/lib/python2.7/site-packages/testtools/runtest.py", line 80, in run
    return self._run_one(actual_result)
  File "/home/user/example/env/local/lib/python2.7/site-packages/testtools/runtest.py", line 94, in _run_one
    return self._run_prepared_result(ExtendedToOriginalDecorator(result))
  File "/home/user/example/env/local/lib/python2.7/site-packages/testtools/runtest.py", line 121, in _run_prepared_result
    result.stopTest(self.case)
  File "/home/user/example/env/local/lib/python2.7/site-packages/testtools/testresult/real.py", line 1271, in stopTest
    return self.decorated.stopTest(test)
  File "/home/user/example/env/local/lib/python2.7/site-packages/testtools/testresult/real.py", line 835, in stopTest
    return self._dispatch('stopTest', test)
  File "/home/user/example/env/local/lib/python2.7/site-packages/testtools/testresult/real.py", line 811, in _dispatch
    for result in self._results)
  File "/home/user/example/env/local/lib/python2.7/site-packages/testtools/testresult/real.py", line 811, in <genexpr>
    for result in self._results)
  File "/home/user/example/env/local/lib/python2.7/site-packages/testtools/testresult/real.py", line 1271, in stopTest
    return self.decorated.stopTest(test)
  File "/home/user/example/env/local/lib/python2.7/site-packages/sst/results.py", line 41, in stopTest
    % self._delta_to_float(elapsed_time))
TypeError: _delta_to_float() takes exactly 3 arguments (2 given)

Fatal error: local() encountered an error (return code 1) while executing 'sst-run -x -s -r xml'