NullPointerException when running XSLTransformer

System

I have implemented a URL external task in Java (using CSSDK) that performs an XSL transformation.

The code is as follows:

*BEGIN CODE
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerFactory;
...
StreamSource transform = new StreamSource(xslFileInputStream);
TransformerFactory tf = TransformerFactory.newInstance();
Transformer t = tf.newTransformer(transform);
*END CODE

I have verified that xslFileInputStream is a valid file, and that it contains what I expect. The problem is, the call "tf.newTransformer()" returns null, which according to the API documentation should never happen.

Has anyone come across this issue before?

Thanks,
Dhruva