Riven not feared by Hecarim

I was playing a game as Hecarim and 4 seperate times I ulted Riven....she didnt get feared a single time. Idk if its a bug with her championship skin, hecarims base skin, or anything like that. A couple of the times Riven used her dash right as I ulted, so maybe if she uses her E during hec's ult she's not being feared?? Idk. All i do know is that 4 different times Riven shouldve gotten feared and killed by my ult, but she didnt. She just kept on fighting as if i never used my ult at all. I got screwed out of atleast 3 kills on her from this happening