I am getting following error on Magento 2.2 after upgrading from 2.1.9

1 exception(s):
Exception #0 (InvalidArgumentException): Unable to unserialize value.

The string on which this error is occuring is as below
a:6:{s:4:"type";s:44:"Amasty\Xlanding\Model\Rule\Condition\Combine";s:9:"attribute";N;s:8:"operator";N;s:5:"value";s:1:"1";s:18:"is_value_processed";N;s:10:"aggregator";s:3:"all";}

Any help us greatly appreciated!