Checkbox Undefined index error in Plugin

I am getting some errors when trying to create a checkbox within a simple plugin I am creating. I have a simple settings page at the moment. The errors are:

Illegal string offset 'single_images' and Undefined index: single_images_zoom

Here is the checkbox code. Any help would be appreciated.

<input id="iwmp_settings[single_images]" name="iwmp_settings[single_images]" type="checkbox" value="1" <?php checked(1, $iwmp_options['single_images'], false); ?> checked="checked" />
<label class="description" for="iwmp_settings[single_images]"><?php _e('Use with Single Images', 'iwmp_domain'); ?></label>

Thanks