Behavior of document() Function with Empty String

The document() function is a source of never-ending confusion for me.  As I
understand it, the document() function should return the root node of the
"current" stylesheet (the one which invoked the function).  Yet when I call
it from within my Java web application, it causes an exception:

org.apache.xml.utils.URI$MalformedURIException: Cannot initialize URI with
empty parameters.
        at org.apache.xml.utils.URI.initialize(URI.java:405)
        at org.apache.xml.utils.URI.<init>(URI.java:229)
        at org.apache.xml.utils.URI.<init>(URI.java:212)
        at
org.apache.xml.utils.SystemIDResolver.getAbsoluteURI(SystemIDResolver.java:1
48)
        at
org.apache.xpath.SourceTreeManager.resolveURI(SourceTreeManager.java:197)
        at
org.apache.xalan.templates.FuncDocument.getDoc(FuncDocument.java:259)
        at
org.apache.xalan.templates.FuncDocument.execute(FuncDocument.java:218)
        at
org.apache.xpath.axes.FilterExprWalker.setRoot(FilterExprWalker.java:158)
        .....

Any explanation or advice?  Links to some definitive sources of info on how
it *should* work and how it works in reality with various parsers would be
appreciated.

--
Roger L. Cauvin
r o g e r @ c a u v i n . o r g
http://cauvin.blogspot.com