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Ed,
It could also be a timing thing whereby, in 6.0, the information for the doc_type_dir() command doesn't have the doctype data available yet. The order in which ACL files are loaded is critical at document loading. When things settle out, you have all the data available, such as when you use the command line.
Or, as Adrian mentions, there may be something prior to the command that is not quite correct that hoses up the response.
Bob