mc_bin_client method getRev doesnt have a consistant behaviour

I am trying to fix the xdcr tests to run for more than 1000 items for whic I am using mc_bin_client.getRev function which is not behaving consistantly.

While running the following code snippet , I get the error "mc_bin_client.MemcachedError: Memcached error #1 'Not found': for vbucket :29 ".

mc = MemcachedClient("127.0.0.1",12000)
mc.vbucket_count = 64
mc.set("a", 0 , 0 , "1")
mc.set("b", 0 , 0 , "2")
mc.set("c", 0 , 0 , "3")
mc.set("d", 0 , 0 , "4")
mc.set("d", 0 , 0 , "5")
print mc.getRev("a")
#print mc.getRev("b")
#print mc.getRev("c")

Stack Trace is as following

File "<stdin>", line 1, in <module>
File "MyTests.py", line 45, in <module>
MCClientTest.test2()
File "MyTests.py", line 39, in test2
print mc.getRev("a")
File "mc_bin_client.py", line 226, in getRev
(meta_type, flags, meta_data) = self.getMeta(key)
File "mc_bin_client.py", line 221, in getMeta
parts=self._doCmd(memcacheConstants.CMD_GET_META, key, '')
File "mc_bin_client.py", line 128, in _doCmd
return self._handleSingleResponse(opaque)
File "mc_bin_client.py", line 121, in _handleSingleResponse
cmd, opaque, cas, keylen, extralen, data = self._handleKeyedResponse(myopaque)
File "mc_bin_client.py", line 117, in _handleKeyedResponse
raise MemcachedError(errcode, rv)
mc_bin_client.MemcachedError: Memcached error #1 'Not found': for vbucket :29 to mc 127.0.0.1:12000