This is a known issue but I believe it is corrected in the latest version in the repository. Try downloading and installing 1.4.3 again.
Previously I was using 1.4.1. I upgraded to 1.4.3 and now it allows me to leave a integer field blank. However it no longer complains if I enter something like "g". Version 1.4.1 did.
Andy
I just tried on my local dev server and a āgā gives an "is not a valid number" error while an empty field is fine..
Care to post your form template and snippet call?