TurboTax allows you to enter an ISO exercise with a "blank" number of shares and does not give an error message. It's not that I left the number of shares blank for the ISO exercise entry; I actually entered a number of shares of 225,000 and TurboTax allowed me to continue with no error message. TurboTax did not alert me that the two grants I entered were actually entered with "blank" shares.
It wasn't until I discovered that TurboTax was not accepting the number of shares that I went directly to the Form 3921 and tried to enter the number of shares there directly. It was then, when I tried to enter the shares directly into the form that I was made aware that there was a maximum number of 5 digits.
When it comes to the AMT calculation, this is a huge deal. The sad part is that TurboTax allows you to enter no shares and the ISO exercise is not used in the AMT calculation. With the IRS, ignorance is not bliss.
I was obviously able to work around the 5 digit maximum once I knew that I had to. Beware if you are entering ISO exercises with shares of more than 5 digits!!!
