Fatal error: Call to a member function getBackendModel() … during Customer Import

Magento Customer Import sometimes throws an error:
“Fatal error: Call to a member function getBackendModel() on a non-object in app/code/core/Mage/ImportExport/Model/Import/Entity/Customer.php on line 411”

If you encounter the same problem, you need to make sure that your custom customer attributes are assigned to a set. Otherwise the following code will remove your attributes from the array which will trigger the above error during the import:

if (!$attribute->isInSet($setId)) {
    unset($attributes[$code]);
}