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The XPath expression test="(@kind cast as messageTypeString)='string'" refers to a type that is not present in the static context for the expression. This is a static XPath error; therefore the schema is invalid. (It is tagged in the metadata as valid; the comment in the test appears to concede that this is incorrect.)
changing metadata to schema is invalid, and remove the instance test.
Resetting the 'component' field of this bug report appropriately.