Fatal error: Call to a member function toOptionArray() on a non-object

Fatal error: Call to a member function toOptionArray() on a non-object in /home/minha-loja/public_html/app/code/core/Mage/Adminhtml/Block/System/Config/Form.php on line 284

Para solucionar esse problema entre no arquivo indicado acima e localize

if ($e->source_model) {
$sourceModel = Mage::getSingleton((string)$e->source_model);
if ($sourceModel instanceof Varien_Object) {
$sourceModel->setPath($path);
}
$field->setValues($sourceModel->toOptionArray($fieldType == 'multiselect'));
}

troque por

if ($e->source_model) {
$sourceModel = Mage::getSingleton((string)$e->source_model);
if ($sourceModel instanceof Varien_Object) {
$sourceModel->setPath($path);
}
if(is_object($sourceModel)){
$field->setValues($sourceModel->toOptionArray($fieldType == 'multiselect'));
} else {
Mage::log($e->source_model);
}
}


fonte: indianicorange.wordpress.com/2010/10/04/fatal-error-call-to-a-member-function-tooptionarray-on-a-non-object/