java.lang.UnsupportedOperationException: Cannot copy a variable reference whose binding is unknown
        at net.sf.saxon.expr.GlobalVariableReference.copy(GlobalVariableReference.java:43)
        at net.sf.saxon.expr.AdjacentTextNodeMerger.copy(AdjacentTextNodeMerger.java:186)
        at net.sf.saxon.expr.Atomizer.copy(Atomizer.java:348)
        at net.sf.saxon.expr.AtomicSequenceConverter.copy(AtomicSequenceConverter.java:290)
        at net.sf.saxon.expr.SystemFunctionCall.copy(SystemFunctionCall.java:306)
        at net.sf.saxon.expr.instruct.ValueOf.copy(ValueOf.java:218)
        at net.sf.saxon.style.XSLTemplate.compileTemplateRule(XSLTemplate.java:731)
        at net.sf.saxon.style.XSLTemplate.compileDeclaration(XSLTemplate.java:648)
        at net.sf.saxon.style.PrincipalStylesheetModule.compile(PrincipalStylesheetModule.java:1242)
        at net.sf.saxon.style.Compilation.compilePackage(Compilation.java:322)
        at net.sf.saxon.style.StylesheetModule.loadStylesheet(StylesheetModule.java:252)
        at net.sf.saxon.style.Compilation.compileSingletonPackage(Compilation.java:113)
        at net.sf.saxon.s9api.XsltCompiler.compile(XsltCompiler.java:838)
        at net.sf.saxon.Transform.doTransform(Transform.java:753)
        at net.sf.saxon.Transform.main(Transform.java:82)
Fatal error during transformation: java.lang.UnsupportedOperationException: Cannot copy a variable reference whose binding is unknown