CXTPPropertyGridItemEnum drop down not appearing

I modified the standard CXTPPropertyGridItemEnum example to load constraints from an array of CStrings.  While I can see all the constraints if I use the arrow keys, clicking on the drop-down button doesn't show the constraint list as expected.  Here is my code sample

   CXTPPropertyGridItem* pOperationItem = pPropertyCategoryItem->AddChildItem(
      new CXTPPropertyGridItemEnum( _T("Operation:"), selectedOperation, &selectedOperation ) );

   for( int i = 0; i < OPERATION_MODE_COUNT; i++ )
   {
      pOperationItem->GetConstraints()->AddConstraint( OPERATION_MODES , i );
   }

Notes:
pPropertyCategoryItem is a CXTPPropertyGridItem* (actually a CXTPPropertyGridItemCategory)
OPERATION_MODES is a CString[]
OPERATION_MODE_COUNT = 5

Please let me know what I am missing.

Thanks in advance!

Sorry, it looks like the posting mechanism ate my [i].  I am doing what you suggested.  I can see the items individually if I use the arrow keys, but I can't see the drop-down menu.  Any other ideas?