SBM API issue - invalid field

I'm trying to use the SBM web services API to transition a ticket. We have an extended field that is required in this transition. If I'm not changing the value (i.e. I am specifying the same value in the API call as is already on the ticket), I get the following and the transition fails: "One or more fields are invalid. Please supply a value for the 'FIELDNAME' field". Any ideas?

Using Version 2009 R4.01.22