ForUpdateFragment.toString calls Dialect.getForUpdateString(aliases, lockOptions), however none of available Dialect implementations overrides this method to support FOR UPDATE OF clause (by default it return FOR UPDATE without OF).

As a workaround you can extend your Dialect implementation and override getForUpdateString method.

@Override
public String getForUpdateString(String aliases, LockOptions lockOptions) {
    return super.getForUpdateString(aliases, lockOptions) + " of "
        + aliases;
}