Not sure what is causing this!

I added a add to cart button to the whats new sidebox using the following code:

    $content .= '<a href="' . zen_href_link($_GET['main_page'], zen_get_all_get_params(array('action')) . 'action=buy_now&products_id=' . $random_whats_new_sidebox_product->fields['products_id']) . '">' . zen_image_button(BUTTON_IMAGE_BUY_NOW_SMALL, BUTTON_BUY_NOW_SMALL_ALT, 'class="sideBoxContentItemBuyNowImage"') . '</a>';
I changed admin setting ADMIN > CONFIGURATION > MY STORE > Display Cart After Adding Product to "false"

It works on every page except the product info page, it adds correct product to cart but returns "Sorry, the product was not found."

Any ideas!