IIS 8：自定义应用程序池ID：值不在预期范围内

I have a pair of IIS 8 machines, both with identical app hosts, both with precisely the same machine keys. On one of them, when I try to use a custom account for my application pool id, everything works hunky-dory. On the other, when I attempt to do this, I close the dialogue box to enter the password, leaving me at the advanced settings dialogue box. When I close that, I receive an error stating "Value does not fall within the expected range."I have a pair of IIS 8 machines, both with iden