A dumb question no doubt - my apologies

Here's the snipet of my code

CD1.Filter = "All Files (*.txt)"
CD1.ShowOpen
If CD1.FileName = "" Then Exit Sub
myfile = CD1.FileName

The problem is when I try to re-open the CDialogue box to add more files ... I get an "Invalid filename" error ... leading me to the CD1.showopen command.

What am I doing wrong?