Unique constraint erroneously finds match when grouped property value is null

I have a user class where I would like the user id to be unique for records marked as not deleted
As a simplification I have the rstriction uniqueness on date deleted (2 deleted records at the same time should also not have the same user id):

class User {
String userId
Date dateDeleted

static constraints =

}

However, if I save a user with userId = "User1" and a not null date deleted value
Then validate a 2nd user with userId = "User1" and a null date deleted, the unique constrain kicks in.

The problem appears in to be in org.grails.datastore.gorm.validation.constraints.UniqueConstraint

line 69:

 final existing = constraintOwningClass.createCriteria().get {
                    eq constraintPropertyName, propertyValue
                    for(prop in group) {
                        eq prop, target[prop]
                    }
                }

evaluates to:

 final existing = User.createCriteria().get {
                    eq 'userId', 'User1'
                    eq 'dateDeleted', null
                }

Which is returning a row, even though the saved user has a not null dateDeleted. This is causing the uniqueness to fire