Magento rewrite Mage_Cms_Helper_Wysiwyg_Images, class not found fatal error

I'm trying to rewrite the Mage_Cms_Helper_Wysiwyg_Images class, but keep hitting a class not found fatal error.

config.xml:

<?xml version="1.0"?>
<config>
    <modules>
        <Namespace_Module>
            <version>0.1.0</version>
        </Namespace_Module>
    </modules>
    <global>
        <helpers>
            <wysiwyg_images>
                <class>Namespace_Module_Helper</class>
            </wysiwyg_images>
            <cms>
                <rewrite>
                    <wysiwyg_images>
                        Namespace_Module_Helper_Wysiwyg_Images
                    </wysiwyg_images>
                </rewrite>
            </cms>
        </helpers>
    </global>
</config>

I have checked several times that the class Namespace_Module_Helper_Wysiwyg_Images exists in app/code/local/Namespace/Module/Helper/Wysiwyg/Images.php and that it extends Mage_Cms_Helper_Wysiwyg_Images.

It seems that the rewrite element is correct, as Magento is attempting to instantiate Namespace_Module_Helper_Wysiwyg_Images, it just can't find it.

Other classes from the module (not shown in the config.xml above) are loaded as expected.

What am I doing wrong that's causing Magento not to load my Namespace_Module_Helper_Wysiwyg_Images class?