Monks, Fists as Natural Weapons, Improved Natural Attack feat

[sloejack]

Hello, I am playing a monk in a 3.5 game and am trying to follow a build that I thought looked fun as the DM is playing with some psychic and psionic beasties in his world. So I'm trying to build towards a Monk/Psionic Fist combo and the build guide I'm following suggests picking up Improved Natural Attack at lvl 6. We're leveling to 6 after the weekends adventure and so when I went to try and add it, it doesn't allow me to select my fists as a valid natural weapon.

So my question is this. Am I following a busted build, or am I using the PC builder wrong in some way?