MaskedTextBox.AsciiOnly Property is not working

Question

I'm using Visual Studio 2005 and have a Form with a MaskedTextBox.
In the Property Grid, i set AsciiOnly to true. All other properties
are unchanged. Despite that, when I run the program, the MaskedTextBox
still accepts numerals and commas, etc... To my knoledge when the AsciiOnly
is set to true, the MaskedTextBox is supposed to restrict input to only A-Z and a-z.
It appears that the AsciiOnly property is not working.

Am I missing something here?