Skeletal Arsonist Command bug

The command "Explosive Demise" for the skeletal arsonist skill is not working properly. The command allows the arsonists to explode based on being at a low threshold of health for big damage to the targets they are near. It has a meter that counts down as you explode them. So when one is low it works and the meter counts down by one each time you can successfully explode one. However, if none of the minions are at the low health threshold they will not explode and the meter will not move BUT the mana is deducted anyway from using the command. So no explosion but mana deducted anyway. Please fix it so that if no minions can be exploded the skill cannot be pressed or mana not deducted without resulting explosion occurring. Thanks for your time.