I guess you've imported just one icon, BTNyour_hero icon. However, you should alsways import two icons per unit/ability/other stuff.
One as BTN when ability is enabled/unit stays alive and DISBTN for cases when ability is disabled/unit is dead. Similar situation happens to all other resources like upgrades ect.
So, you have to create a Disabled icon of your hero's icon, do it via any border icon program found in tools. Import both icons, make sure they got same path, difference is 'prefix' DIS in disabled icon.