I'm trying to add a reference from the default Spring Security user domain class to another class which should hold additional user info. class User { transient springSecurityService String username String password boolean enabled boolean accountExpired boolean accountLocked boolean passwordExpired Profile profile ... ...and... class Profile { static constraints = { } static belongsTo = [user : User] String firstName String lastName } From what I read this should be the way it works, but I get the following error : ERROR context.GrailsContextLoader - Error executing bootstraps: null Message: null Line | Method ->> 32 | create in com.app.UserRole - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - | 15 | doCall in BootStrap$_closure1 | 301 | evaluateEnvironmentSpecificBlock in grails.util.Environment | 294 | executeForEnvironment in '' | 270 | executeForCurrentEnvironment . . in '' | 303 | innerRun in java.util.concurrent.FutureTask$Sync | 138 | run . . . . . . . . . . . . . . in java.util.concurrent.FutureTask | 886 | runTask in java.util.concurrent.ThreadPoolExecutor$Worker | 908 | run . . . . . . . . . . . . . . in '' ^ 662 | run in java.lang.Thread Any idea? ======================================================================= We need to see your BootStrap to be sure, but I suspect it's because you're doing something like def user = new User(....).save() def role = new Role(....).save() UserRole.create(user, role) and your new User is failing validation (which causes save to return null). If you use save(failOnError:true) you should get a different exception with a better indication of what the real problem is. Check that you have the right constraints in your User class, in particular note that GORM properties are by default non-nullable so if you want to be able to save a User that doesn't have a Profile you will need to add a constraint of profile(nullable:true).