SAP Message BK185 - Document with external ref. cannot be reversed through internal number.

Diagnosis You want to reverse a CO document which has an external reference. Youcannot reverse the external document within the Controlling component.The reversal must run via the external document number. System Response The R/3 System does not reverse the document. Procedure Reverse the document by way of the external reference, where possiblewith a reference for the internal reversal as well.