Adding extra fields from the type 'textarea' causes an error with mb_strlen() when submitting form

Hi

I needed to add a few fields to the default ones on the theme_signup_user_form() function, from which two are of the type 'textarea'.
So no, my theme_signup_user_form() looks a bit like this:

function theme_signup_user_form() {
global $user;

// This line is required for this form to function -- DO NOT EDIT OR REMOVE.
$form['signup_form_data']['#tree'] = TRUE;

$form['signup_form_data']['Name'] = array(
'#type' => 'textfield',
'#title' => t('Name'),
'#size' => 40, '#maxlength' => 64,
'#required' => true,
);

$form['signup_form_data']['Nationality'] = array(
'#type' => 'textfield',
'#title' => t('Nationality'),
'#size' => 40, '#maxlength' => 64,
);
$form['signup_form_data']['Address'] = array(
'#type' => 'textarea',
'#title' => t('Address'),
'#cols' => 40, '#rows' => 4,
);
$form['signup_form_data']['Email'] = array(
'#type' => 'textfield',
'#title' => t('Email'),
'#size' => 40, '#maxlength' => 64,
'#required' => true,
);

$form['signup_form_data']['Other'] = array(
'#type' => 'textarea',
'#title' => t('Other required entries - check event description'),
'#cols' => 40, '#rows' => 6,
);

// If the user is logged in, fill in their name by default.
if ($user->uid) {
$form['signup_form_data']['Name']['#default_value'] = $user->name;
}
return $form;
}

The problem is that every time someones registers for an event, I get an error:
"mb_strlen() expects parameter 1 to be string, array given in includes/unicode.inc on line 370."

I have to confess that I'm not an expert, and this is probably not a bug in the signup module per se, but I would like to know if I can correct this (and how) or if this is indeed a bug in the signup module.

Oh, and I only used the type 'textarea' because it's the only one that I'm aware of that has multiple line input.

Best regards