Use attached to reproduce.

1 - In the First Text Box (the Masked Box) type 1
2 -  Click the "GO" button. The text box shows 1 and the VALUE is displayed from the Edit box correctly as 1
3 - Select the 1 in the edit box and CTRL + DELETE to clear the field
4 - Click the "GO" button. The text box shows NULL and the VALUE is displayed from the Edit box correctly as NULL
5 - Highlight the text in the "Paste Me!" text box and Copy the text to Clipboard.
6 - Paste the clipboard into the Masked t Edit box
7 - Click "GO". The mask text box correctly displayed NULL however the returned VALUE is 1