Memchached error #4 when merging a backup

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What's happening
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A stop persistence call (0x80) returns the following error:
File "lib/mc_bin_client.py", line 659, in stop_persistence
return self._doCmd(memcacheConstants.CMD_STOP_PERSISTENCE, '', '')
File "lib/mc_bin_client.py", line 229, in _doCmd
return self._handleSingleResponse(opaque)
File "lib/mc_bin_client.py", line 220, in _handleSingleResponse
cmd, opaque, cas, keylen, extralen, data = self._handleKeyedResponse(myopaque)
File "lib/mc_bin_client.py", line 216, in _handleKeyedResponse
raise MemcachedError(errcode, msg)
mc_bin_client.MemcachedError: Memcached error #4 'Invalid': b''

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What I expect
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I expect it to stop persistence

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Steps to reproduce
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I can't seem to understand how to reproduce it outside a backup test in test runner. I've attached cbcollectlogs logs.

To run the test itself:

./testrunner -i {PATH_TO_INI} -p get-coredumps=False,skip_log_scan=False,bucket_storage=couchstore -t ent_backup_restore.enterprise_backup_merge_tests.EnterpriseBackupMergeTest.test_multiple_backups_merges,objstore_provider=s3,actions="backup_with_ops:2,backup:1,rollback,backup_with_ops:1,backup:1,backup_with_ops:2,backup:1,rollback,backup_with_ops:1,backup:1",nodes_init=3,GROUP=P1,get-coredumps=False,skip_log_scan=False,bucket_storage=couchstore

I couldn't identify the exact build it's server started because there were other errors that were shadowing this one.