Error in approving Purchase Requisition if the optional note is not filled

Hi,

When we're approving PR with custom document type and not fill-in the optional note, we receive the error shown below.

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Clik here to view.

Note:

- Approving of PR works in the backend via ME52N even w/o the note.

- Approval thru Fiori App works when approving standard document type.

Thanks for the help in advance!