The current code for XPathUtil.JstlVariableContext.notNull looks like:

        private Object notNull(Object o, String prefix, String localName)
        throws UnresolvableException {
            if (o == null) {
                throw new UnresolvableException("$" +
(prefix==null?"":"prefix"+":") + localName);
            }
            //p("resolved to: " + o);
            return o;
        }

Which means that if the prefix arg is non-null, the exception message will
always include the literal string "prefix" as opposed to the arg's value. The
fix is to do something like:

        private Object notNull(Object o, String prefix, String localName)
        throws UnresolvableException {
            if (o == null) {
                throw new UnresolvableException("$" +
(prefix==null?"":prefix+":") + localName);
            }
            //p("resolved to: " + o);
            return o;
        }

I can't create a diff/patch while at work, but the fix is pretty simple. I'll
try to upload a diff tonight unless somebody else beats me to it...