supertype "nt:unstructured" not found

On first startup with the usage of the jpa-connector-store into a mysql an app creates a own nodetype, here "brix:node"; Then system is shutdown regularly and restarted. Upon startup ERROR is listed as follows:

ERROR - RepositoryNodeTypeManager - Node types were read from the system content, and appear to be inconsistent or invalid: org.modeshape.jcr.nodetype.InvalidNodeTypeDefinitionException: Supertype 'nt:unstructured' from type 'brix:node' does not exist|#]

It doesn't have any impact on the work so far (at least I couldn't find anything yet), but the mentioned supertype is from JCR API definition and therefore of course not defined by the app;