XSLT Map error: btm1050

error btm1050: XSL transform error: Unable to write output instance to the following <output.xml>. Value cannot be null.
Parameter name: extension

You what now? This error was being produced when testing a BizTalk map that uses XSLT and XML Extensions (i.e. external assemblies). Debugging the map produces a slightly different error:

XslTransformException
---------------------
Cannot find a script or an extension object associated with namespace 'http://whatever/your/namespace/is'.

Where the namespace relates to the first called bound namespace for external assemblies.

This one took me a long time to figure out. In this situation there is a shared extensions XML file with a bunch of possible classes that are used in various different XSLT maps. The error was being produced by a reference to a class that wasn't bound or in use by the XSLT map - this class had been refactored to be abstract. The error I assume was happening when the class was trying to be instantiated - obviously something you can't do when it's abstract.

Removing these old class references from the extensions XML solved all the problems.