form field name="key[]" causes "Disallowed Character Keys"

[eluser]skunkbad[/eluser]
I've got a series of form fields that must be posted as an array, but CI doesn't seem to like the square brackets. I'd hate to hack the input class. What's the deal with this type of form field?

Code:

<td>&lt;input type="text" name="whatever[]" value="" /&gt;&lt;/td>

It's just standard HTML.