How do you know it's a valid conclusion? Applying reason? Watch out for that Jacobinism bugbear.
If you can "reason" that there are limits to human reason, does that mean that reason is still king? Or does it mean that it's merely a useful stepping stone along the way?
It means you probably shouldn't make universal pronouncements about the inapplicability of rationalism to the political sphere... ;)
Are we now equating rationalism with reason? If I'm not mistaken, rationalism is a philosophy which demands the predominance of reason in guiding our affairs - which, as you seem to agree (as you've taken it as the premise for your statement above), is self-contradictory. The contrary position is not that reason has no place, but that it has a diminished place. So therefore, I would correct your statement thus:
"It means you probably shouldn't make universal pronouncements about the applicability of reason to the political sphere"