Incorrect Rune reward for Myrtoan Sea quests in Defense of Crete?

I suspect that the 10/20/30 wave Myrtoan Sea quest should give a Rune of Svartalheim reward, but instead gives the Rune of Vanaheim reward. But the Ieropotamas River quest already gives the Rune of Vanaheim reward. So....just wondering how would I get the Rune of Svartalheim reward to complete my collection of the 9 Runes?