Unable to evoke using floating mana

I was playing in the Lorwyn Eclipsed Premium Draft and had a copy of Emptiness in my hand. During my first main phase, I proceeded to add 1 green mana and then used Foraging Wickermaw to convert that mana into 1 black mana. I then tapped a swamp for another 1 black mana and had two floating black mana. I then attempted to cast Emptiness for its evoke cost. When the prompt came up on how to pay, I chose 2 black mana but then the game refused to use the black mana I had floating to cast it. I had to go with another play and lost the game. I would like my gems back and please fix this