Pick Two Face consecutively problem

When I pick the first face, an error is thrown saying "The user aborted the pick operation." If I put MessageBox in between picking the first and second face, there are  no errors. Why is this so? Thanks!

Error occur when i write the code as below:

Reference pickedRef01 = sel.PickObject(ObjectType.Face, "Please select a Face");

Reference pickedRef02 = sel.PickObject(ObjectType.Face, "Please select a Face");

Reference pickedRef01 = sel.PickObject(ObjectType.Face, "Please select a Face");

MessageBox.Show("Select Face Please");

Reference pickedRef02 = sel.PickObject(ObjectType.Face, "Please select a Face");