Could not initialize db object for request

HI i got this error when i process "completion" 

Parameters

Array
(
    [type] => completion
    [txn_number] => 20228-0_10
    [order_id] => 6371
    [comp_amount] => 100.00
    [crypt_type] => 7
    [cust_id] => 55635
)

got this response
[Message] => sendMessage(): Merchant - Could not initialize db object for request.

by the way where can i get the txn_number from pre-authorization? is that a unique number from moneris pre-auth? 

thanks..