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.