Database migration fails for unique index on two fields one of which is the object owner

Given a domain object with a unique index on two fields - one of which is the object owner - I systematically get an error message executing the generated changelog on a h2 database. It complains:

dbcSQLException: Syntax error in SQL statement "CREATE INDEX UNIQUE[*]-CHILD_POS ON FOO(PARENT_ID, CHILD_POS) "; expected "identifier"; SQL statement:
CREATE INDEX unique-child_pos ON foo(parent_id, child_pos) [42001-164]

Here's the relevant portion of the changelog

...
    changeSet(author: "davide (generated)", id: "1337257962240-2") {
        createIndex(indexName: "unique-child_pos", tableName: "foo") {
            column(name: "parent_id")
            column(name: "child_pos")
        }
    }
...

It seems like the plugin "somewhat misread" the 'unique' keyword, generating an invalid indexName (unique-child_pos). I guess it would be quoted when containing chars other than: A-Z>|_ (see h2 sql grammar).

The domain object code is

package grails_test

class Foo {
    int childPos = -1
    static belongsTo = [parent: Foo]
    static mapping = { sort 'childPos' }
    static constraints = {
        parent (nullable: true)
        childPos (unique: ['parent'])//childPos is unique (given a certain parent)
    }
}

Btw grails_test.zip is a complete test project.