node_import_check_node_reference having no matching reference

Using a node reference field and the is no matching node to this title
the importer throws

node_import_input_error(
    t('Input error: %value is not allowed for %name (not a node reference).', 
    array('%value' => $value, '%name' => $field['title'])));

(node_import.inc :: node_import_check_node_reference, line 1281)

But there are node references that are not mandatory. So this code rejects importing those items.
I commented out this line without having trouble.

I think there should only be this error thrown when this reference is mandatory.
Otherwise this item should be imported without a reference.