Something seems odd with your database.
However, this bandage approach may work:

/admin/coupon_restrict.php
around line 286 you have:
Code:
                    <form name="restrict_category" method="post" action="<?php echo zen_href_link('coupon_restrict.php', zen_get_all_get_params(array('info', 'action', 'x', 'y')) . 'action=add_product&info=' . $cInfo->restrict_id, 'NONSSL'); ?>">
                    <td class="smallText" valign="top"><?php echo HEADER_PRODUCT_NAME; ?></td>
                    <td class="smallText" align="left"><?php echo zen_draw_pull_down_menu('products_drop', $products_array, $current_category_id); ?></td>
                    <td class="smallText" align="left"><?php echo zen_draw_pull_down_menu('restrict_status', $restrict_array); ?></td>
                    <td class="smallText" align="left"><input type="submit" name="add" value="Add"></td>
                    <td class="smallText" align="left">&nbsp;</td>
                    <td class="smallText" align="left">&nbsp;</td>
change that to:
Code:
<?php if (sizeof($products_array) > 0) { ?>
                    <form name="restrict_category" method="post" action="<?php echo zen_href_link('coupon_restrict.php', zen_get_all_get_params(array('info', 'action', 'x', 'y')) . 'action=add_product&info=' . $cInfo->restrict_id, 'NONSSL'); ?>">
                    <td class="smallText" valign="top"><?php echo HEADER_PRODUCT_NAME; ?></td>
                    <td class="smallText" align="left"><?php echo zen_draw_pull_down_menu('products_drop', $products_array, $current_category_id); ?></td>
                    <td class="smallText" align="left"><?php echo zen_draw_pull_down_menu('restrict_status', $restrict_array); ?></td>
                    <td class="smallText" align="left"><input type="submit" name="add" value="Add"></td>
                    <td class="smallText" align="left">&nbsp;</td>
                    <td class="smallText" align="left">&nbsp;</td>
<?php } else { ?>
                    <td class="smallText" align="left" colspan="6">&nbsp;</td>
<?php } ?>