VS2010 WPF DataGrid Sorts Improperly

VS2010 WPF DataGrid Sorts Improperly

I am using a WPF DataGrid with procedurally (C#, VS 2010) generated, bound
columns, and the DataGrid will not sort data correctly.
CanUserSortColumns is set to true. SortMemberPath is set to the same
property as the text displayed in the grid.
Regardless of which column the user sorts, and despite the sort icon being
displayed over the appropriate column, the DataGrid merely alternates the
sort order of the first column.
column.Header = departmentColumn.ColumnHeader;
column.Width = departmentColumn.ColumnWidth;
column.Binding = new Binding("Cells[" +
departmentColumn.Ordinal.ToString() + "]");
column.SortMemberPath = "DisplayString";
I have no problems with any other data being displayed or used incorrectly
by the DataGrid, so I am stumped. Why would the sort only consider text in
the first column, when everything else binds to data from the appropriate
column?