Dynamics GP: The Customer Priority parameter is invalid

An existing working integration to Dynamics GP resulted in the following error when attempting to update a specific Customer record in Dynamics GP:

Status Message: cli.Microsoft.Dynamics.GP.eConnect.eConnectException: Sql procedure error codes returned: Error Number = 6328 Stored Procedure taUpdateCreateCustomerRcd Error Description = The Customer Priority parameter is invalid: 1 to 100 are valid values.

Details on the integration:

• Dynamics GP 2010, SQL Server 2008, eConnect 11/2010
• The integration wasn’t using or passing the Customer Priority parameter
• This Customer record had been previously updated successfully by the integration
• This specific Customer caused the error, different Customer records could be updated without error

Analysis of the Customer record in GP showed that the Priority field was blank, Cards > Sales > Customer > Priority:

Anyone familiar with Dynamics GP, will know that the Customer Priority field is a dropdown and that blank is not a valid value for this field:

The solution was to edit the Customer in Dynamics GP and select a specific value from the drop-down box and save the customer record.  Everything worked fine after this so we concluded that a blank or invalid field was previously in the database for this Customer, causing the integration to fail.