Gild unable to cast on hero specials +other query

Unable to cast Gild on any named special hero ie: Hand of Kira, Lord O'Theland, Chippy and my newly aquired Rousslmoff. It warns me of insufficient mana whilst using below 500 mana, says nothing when above 500. Tooltip doesn't have any indication of mana costs. Works on all other minions; including heroes.

EDIT: I've added this query here; so to not have too many threads.

I foolishly added a rift here out of curiosity however my imps only randomly get to these islands and haven't built it as of yet, also I doubt the demons can even travel across the chasm to do my bidding. The issue I face is I'm unable to sell or cancel the rift, forever locking 75 mana. I loaded an old save so its not hindering me, though it may still be an unintended feature. EDIT#2: Gave it some thought and did the same thing and used construction ritual and was able to sell it; only once it was completed.