transformToXml返回false但libxml_get_last_error（）为空

I am trying to transform an XML file using XSL in PHP. Both the XML and XSL files are loaded correctly from the local file system, but when I call:

$xslt->transformToXml($xml)

it returns false, yet both libxml_get_last_error() and libxml_get_errors() are empty.

Is there a way I can debug this or get some useful errors?

NB. My error_reporting is E_ALL | E_STRICT, display_errors is on.