EM_SETSEL message not highlighting selection
 

What could cause edit control data to NOT display highlighted after I send an EM_SETSEL message? The data IS being selected, I know because if I send a "CUT" message the selection is deleted. This problem occurs with partial field selection and for "Select All". I've used EM_SETSEL in other applications without having this problem.
This is a masm32 program, the edit control is in a modeless dialog box.