What's wrong with this?

broken.php

    protected function _addFacetMappings()
    {
        $sql = <<<SQL
            INSERT INTO `{$this->_db->prefix}solr_search_facets`
                (element_id, name, element_set_id, is_facet, is_displayed, is_sortable)
                VALUES (?, ?, ?, ?, ?, ?);
SQL;
        $stmt = $this->_db->prepare($sql);

        // These get inserted fine.
        $stmt->execute(array(null, 'image',      null, 1, 1, 1));
        $stmt->execute(array(null, 'tag',        null, 1, 1, 1));
        $stmt->execute(array(null, 'collection', null, 1, 1, 1));
        $stmt->execute(array(null, 'itemtype',   null, 1, 1, 1));

        $elements = $this->_db->getTable('Element')->findAll();

        $f = fopen('/vagrant/solr.log', 'w');

        foreach ($elements as $element) {
            // This line produces the output I expect, so it is getting the elements and iterating over them.
            fwrite($f, "\n+++ {$element['id']} => [{$element['element_set_id']}] '{$element['name']}'\n");
            // However, none of these actually get inserted into the database.
            $stmt->execute(array(
                $element['id'], $element['name'], $element['element_set_id'], 0, 0, 0
            ));
        }

        fwrite($f, "DONE\n");
        fclose($f);
    }

working.php

    protected function _addFacetMappings()
    {
        $elements = $this->_db->getTable('Element')->findAll();
        $sql = <<<SQL
            INSERT INTO `{$this->_db->prefix}solr_search_facets`
                (element_id, name, element_set_id, is_facet, is_displayed, is_sortable)
                VALUES (?, ?, ?, ?, ?, ?);
SQL;
        $stmt = $this->_db->prepare($sql);

        $stmt->execute(array(null, 'image',      null, 1, 1, 1));
        $stmt->execute(array(null, 'tag',        null, 1, 1, 1));
        $stmt->execute(array(null, 'collection', null, 1, 1, 1));
        $stmt->execute(array(null, 'itemtype',   null, 1, 1, 1));

        $f = fopen('/vagrant/solr.log', 'w');

        foreach ($elements as $element) {
            // This line produces the output I expect, so it is getting the elements and iterating over them.
            fwrite($f, "\n+++ {$element['id']} => [{$element['element_set_id']}] '{$element['name']}'\n");
            // However, none of these actually get inserted into the database.
            $stmt->execute(array(
                $element['id'], $element['name'], $element['element_set_id'], 0, 0, 0
            ));
        }

        fwrite($f, "DONE\n");
        fclose($f);
    }