Calling an external assembly from a custom xslt file

I am trying to call an external assembly from a custom xslt file that I am referencing in my BizTalk map. However, I get the following error when I try and test the map:

“System.Xml.Xsl.XslTransformException : Cannot find the script or external object that implements prefix….”

The annoying thing here is that I am able to successfully call another assembly and test it but the moment I try and call my new assembly I get the error described above.

The new assembly is in the GAC and I’ve referenced it within the ExtensionObjects XML. The class itself is not static although the methods are.

I have tried to call the assembly that is contained in the Samples that are installed in the BizTalk SDK with the same result.

I must be missing something obvious and am starting to think that it is an environmental issue, however if that was the case then I would expect the older assembly not to work too.

Thanks,

Andrew