Can't replace Greater Iron Rune with Thane Myrk's rune of summer

I bought a 2 div weapon with 2 greater iron runes already in it, I replaced the first rune successfully with Thane Myrk's Rune of summer, but the second one will not replace. no error message, it just does not work. so now my Thunder Talisman has 1 Greater iron and 1 Thane Myrk's Rune of Summer

Special runes are limited to 1. This is on the tooltip.