failed in world_io_test

[mingmou123]

I follow the instruction, but an error occured, what should I do?
=================================== FAILURES ===================================
__________________________ WorldIOTest.testSaveWorld ___________________________

self = <tests.integration.world_io_test.WorldIOTest testMethod=testSaveWorld>

def testSaveWorld(self):
    GraspitCommander.importRobot("Barrett")
    GraspitCommander.importGraspableBody("ashtray")

  GraspitCommander.saveWorld("test_world")

tests/integration/world_io_test.py:59:

---

fileName = 'test_world'

@staticmethod
def saveWorld(fileName):
    _wait_for_service(GraspitCommander.GRASPIT_NODE_NAME + 'saveWorld')

    serviceProxy = rospy.ServiceProxy(GraspitCommander.GRASPIT_NODE_NAME + 'saveWorld', SaveWorld)
    result = serviceProxy(fileName)

    if result.result is SaveWorld._response_class.RESULT_SUCCESS:
        return
    elif result.result is SaveWorld._response_class.RESULT_FAILURE:

      raise SaveWorldException()

E SaveWorldException: Could not save world.

src/graspit_commander/graspit_commander.py:497: SaveWorldException
------------------------------ Captured log call -------------------------------
tcpros_base.py 640 DEBUG [/graspit/clearWorld]: writing header
tcpros_base.py 640 DEBUG [/graspit/importRobot]: writing header
tcpros_base.py 640 DEBUG [/graspit/importGraspableBody]: writing header
tcpros_base.py 640 DEBUG [/graspit/saveWorld]: writing header
=============== 1 failed, 12 passed, 2 skipped in 26.54 seconds ================