Can't use GUID as key field in datagrid?

Question

I have two datagrids, both read and display all columns returned by either a stored procedure or a sql select statement, including a column that contains guids. In both cases, the guid column does not show up in the list of columns that can be used as the Data Key Field in the general tab of the property builder dialog box (visual studio). It also fails to show up as an available column in the columns section of same. Am I missing something? Any ideas?