"Invalid derived query" marker for a delete query

In the following repository declaration STS marks the delete method with an error: "Invalid derived query! No property delete found for type XYZEntity!"

public interface XYXEntityRepository extends MongoRepository<XYZEntity, String> {
    XYZEntity findOneByInstitutionId(String id);
    Long deleteByInstitutionId(String id);
}

The project is configured to use spring-data-mongo 1.5.0.RC1.