Magento: Adding new products programmatically

I am trying to add products to Magento 1.5 programmatically. My script will ultimately be a cron job, regularly updating and adding products as dictated by an XML file supplied by the accounts system.

I have a problem in creating new products. The relevant code segment from my script is:

    $attributeSetId = 4;

    //$newproduct = Mage::getModel('catalog/product');
    $newproduct = new Mage_Catalog_Model_Product();

    $newproduct->setTypeId('simple');
    $newproduct->setWeight($product->UnitWeight);       
    $newproduct->setVisibility(Mage_Catalog_Model_Product_Visibility::VISIBILITY_BOTH); 
    $newproduct->setStatus(1);
    $newproduct->setSKU($SKU);
    $newproduct->setTaxClassId(0);
    $newproduct->setWebsiteIDs(array(0)); 
    $newproduct->setStoreIDs(array(1)); 
    $newproduct->setStockData(array( 
        'is_in_stock' => 1, 
        'qty' => $XMLproduct->QtyInStock,
        'manage_stock' => 1
    )); 

    $newproduct->setAttributeSetId(4);
    $newproduct->setName($product->Name);
    $newproduct->setCategoryIds(array(3)); // array of categories it will relate to

    $newproduct->setDescription($product->LongDescription);
    $newproduct->setShortDescription($product->Description);
    $newproduct->setPrice($XMLproduct->SalePrice);

    try {
        if (is_array($errors = $newproduct->validate())) {
            $strErrors = array();
            foreach($errors as $code=>$error) {
                $strErrors[] = ($error === true)? Mage::helper('catalog')->__('Attribute "%s" is invalid.', $code) : $error;
            }
            $this->_fault('data_invalid', implode("\n", $strErrors));
        }

        $newproduct->save();
    } catch (Mage_Core_Exception $e) {
        $this->_fault('data_invalid', $e->getMessage());
    }

The product is 'half' created, but the script gives up throwing the following error: