Unable to upgrade data from ver 2.1.3 to latest Magento ver 2.2.6

[msh_h]

I am trying to upgrade our Magento (2.1.3) to latest version (2.2.6) but it fails at setup upgrade process of Magento_Sales module's data:

customerId, , customerId, , customerId, , customerId, , customerId, , customerId, , customerId, , customerId, , customerId, , customerId, , customerId, , customerId, , customerId, , Fatal error: Allowed memory size of __ bytes exhausted (tried to allocate __ bytes) in \vendor\magento\framework\Exception\NoSuchEntityException.php on line 51

public static function singleField($fieldName, $fieldValue)
{    

echo $fieldName . ', ' . $fieldValue . ', ';

    return new self(
        new Phrase(
            'No such entity with %fieldName = %fieldValue',
            [
                'fieldName' => $fieldName,
                'fieldValue' => $fieldValue
            ]
        )
    );
}

First, I thought its memory_limit issue, but it took several hours with 4G limit and failed again. So, I printed above in the NoSuchEntityException class and realized its infinite loop with customerId field.

I saw other similar issues about wrong email error, but didn't find anyone having same issue.

Note: Our sales_order table contains only about 4K records and sales_order_address 8K.

Please help! Its driving me crazy and I repeated the upgrade steps multiple times but in vain