Why did I get API error code 11451?

API error code 11451 occurs when a billing agreement ID or transaction ID is not valid. The Reference ID field caused the error for one of the following reasons:

• The field is empty.
• It contains an invalid transaction.
• It can't be decrypted.
• It isn't associated with a billing agreement.
  Note: If the billing agreement ID is correct, inform the customer that a new billing agreement is needed and offer them the chance to sign up for a new one through Express Checkout.

See the Developer Portal for a complete list of NVP/SOAP API error codes.