Datagrid SortMemberPath=Binding gives error Object reference not set

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C#

WPF

sorting

DataGrid

Hello ,

In my datagrid my textColumn looks like:
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<datagridtextcolumn header="Name">
IsReadOnly="True" Width="300"
Binding="{Binding Path=CT_APPR.NAME}"
SortMemberPath="{Binding Path=CT_APPR.NAME}" >


My datagrid looks fine, so the binding is correct.
But when I click on the column-header to sort the column I get the error:
"Object reference not set to an instance of an object."
How can that be, when the correct name is shown in the column?

Is there another way to get the sorting fixed? Sort on the cell-value?

Thank you