I did some testing with this but it was in php. What I found was that the space character was not matching the list of not allowed in the function call.
In php I could get it to work by making the not allowed value , I then had to add a string replace function around the value to replace any space with . When I did this it would pick up the space properly and show the error message when it was present.
There should be an equivalent thing you can do in the asp code. Please post back with a copy of the page that has the validation applied to it along with your WA_ValidationToolkit folder in a zip archive. I will take a look to see if I can get it working this way for you.