1. Pick Nature's Prophet, and level up Nature's Call
2. Get an Iron Talon
3. Summon Treants
4. Select your hero and your treants
5. Select Iron Talon's ability
6. TAB to your treants
7. Target a creep with the still selected Iron Talon ability
8. Now, repeat, but this time target a tree with the ability, instead of a unit

Result: When targeting a unit, the hero still casts the ability on it regularly. However, when the target is a tree, the ability is not used and it gives the "Item Is Not In Selected Unit's Inventory" error message and gets canceled. (also, in case of creep-heroes, instead of giving an error message, the creep-hero moves to the targeted tree and then stands still.)

Expected: Tree-targeting should behave like unit targeting. When having tabbed to another unit, the selected ability should still be cast by the hero on the targeted tree, just like how unit-targeted abilities work.

This goes for all tree targeted abilities. They all give that error message and get canceled. Unit-targeted abilities get cast normally. Iron Talon is just the best example, because it can target both.