run-test.php in php 6

I have a bunch of php 6 tests which work when I run them in non-unicode (php run-test.php -N test.phpt) and in unicode mode (php run-test.php -u test.phpt) but the first test fails when I run it in "both" mode (php run-test.php -U test.phpt). I thought if test passes -N and -u, it should always pass -U too. Anybody encountered such problem before? Any ideas what might be the reason?