rospkg.manifest.InvalidManifest

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when I run roscore,the output is like below,how can I solve this problem?

WARNING: unable to configure logging. No log files will be generated
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

Traceback (most recent call last):
  File "/opt/ros/fuerte/bin/rosversion", line 5, in <module>
    pkg_resources.run_script('rospkg==1.0.2', 'rosversion')
  File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 499, in run_script
    self.require(requires)[0].run_script(script_name, ns)
  File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 1235, in run_script
    execfile(script_filename, namespace, namespace)
  File "/usr/local/lib/python2.7/dist-packages/rospkg-1.0.2-py2.7.egg/EGG-INFO/scripts/rosversion", line 76, in <module>
    version = rosstack.get_stack_version(stack_name)
  File "/usr/local/lib/python2.7/dist-packages/rospkg-1.0.2-py2.7.egg/rospkg/rospack.py", line 359, in get_stack_version
    return get_stack_version_by_dir(self.get_path(stack))
  File "/usr/local/lib/python2.7/dist-packages/rospkg-1.0.2-py2.7.egg/rospkg/rospack.py", line 412, in get_stack_version_by_dir
    m = parse_manifest_file(stack_dir, STACK_FILE)
  File "/usr/local/lib/python2.7/dist-packages/rospkg-1.0.2-py2.7.egg/rospkg/manifest.py", line 343, in parse_manifest_file
    return parse_manifest(manifest_name, f.read(), filename)
  File "/usr/local/lib/python2.7/dist-packages/rospkg-1.0.2-py2.7.egg/rospkg/manifest.py", line 404, in parse_manifest
    m.author = _check('author')(p, filename)
  File "/usr/local/lib/python2.7/dist-packages/rospkg-1.0.2-py2.7.egg/rospkg/manifest.py", line 67, in check
    raise InvalidManifest("Invalid manifest file [%s]: must have a single '%s' element"%(filename, name))
rospkg.manifest.InvalidManifest: Invalid manifest file [/opt/ros/fuerte/share/ros/stack.xml]: must have a single 'author' element
Invalid <param> tag: Cannot load command parameter [rosversion]: command [rosversion ros] returned with code [1]. 

Param xml is <param command="rosversion ros" name="rosversion"/>