Migration fails whenever configuring a bandwidth

Unable to migrate guest:
 Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/migrate.py", line 457, in _async_migrate
    vm.migrate(dstconn, migrate_uri, rate, live, secure)
  File "/usr/share/virt-manager/virtManager/domain.py", line 1387, in migrate
    self._backend.migrate(destconn.vmm, flags, newname, interface, rate)
  File "/usr/lib64/python2.6/site-packages/libvirt.py", line 511, in migrate
    if ret is None:raise libvirtError('virDomainMigrate() failed', dom=self)
libvirtError: internal error unable to execute QEMU command 'migrate_set_speed': Invalid parameter type, expected: number