Ecore diagram cannot resolve type reference 'string'

The following warning is shown when org.eclipse.escet.cif.metamodel/model/cif.genmodel is opened:

The warning is also shown when a new .genmodel is created (File --> New --> Other... --> Eclipse Modeling Framework --> EMF Generator Model --> Enter a name --> Select ECore model --> Browse to cif.ecore --> Click Load).

@ddennis noted that:

Check it for all metamodels. It may need to be String or EString or so instead of string, not sure.