not enough information as to how you encounter this.
- 
Is this transaction trying to update the ID field ?
 
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Are you trying to update an asset (in the same transaction) that is not yet committed to the ledger ? 
 
- Have you got ACLs applied (so the participant in question cannot 'see' the asset in question due to restrictions).
 
Suggest to post the model and the full transaction code / sample data.