Sorting y-value or calculated dimension doesn't work

[Anonymous]

I have a table as shown in the picture.

I want to sort the table on the expression descending ... (column Besteed) ...

First I thougth to use the sort on y-value ... this didn't work ... I guesse since I have a column Omschrijving which is seen as y-value?
When using sort on expression -- and using the same expression as used in expression column Besteed also didn't work (gives the resulting sorting as in the picture above ... ), although I thought this should do the trick.

What am I doing wrong ... I'm staring blind on this one ...

thx in advance.