Memcached returns ambiguous status code when trying to update a locked key

A simple test to see if updates are possible on locked keys

def test_lock(self):
src = MemcachedClient(host=self.src_master.ip, port=11210)

1. first set
   src.set('pymc1098', 0, 0, "old_doc")
2. apply lock
   src.getl('pymc1098', 30, 0)
3. update key
   src.set('pymc1098', 0, 0, "new_doc")

throws the following Memcached error -

File "pytests/xdcr/uniXDCR.py", line 784, in test_lock
src.set('pymc1098', 0, 0, "new_doc")
File "/Users/apiravi/Documents/testrunner/lib/mc_bin_client.py", line 163, in set
return self._mutate(memcacheConstants.CMD_SET, key, exp, flags, 0, val)
File "/Users/apiravi/Documents/testrunner/lib/mc_bin_client.py", line 132, in _mutate
cas)
File "/Users/apiravi/Documents/testrunner/lib/mc_bin_client.py", line 128, in _doCmd
return self._handleSingleResponse(opaque)
File "/Users/apiravi/Documents/testrunner/lib/mc_bin_client.py", line 121, in _handleSingleResponse
cmd, opaque, cas, keylen, extralen, data = self._handleKeyedResponse(myopaque)
File "/Users/apiravi/Documents/testrunner/lib/mc_bin_client.py", line 117, in _handleKeyedResponse
raise MemcachedError(errcode, rv)
MemcachedError: Memcached error #2 'Exists': Data exists for key for vbucket :0 to mc 10.3.4.186:11210