Zilean passive bug

Hello! I recently started playing Zilean, because I get autofilled as support alot and I don't trust my ADC to NOT die... While playing I noticed a weird bug that appears when his passive is up and you try to lvl up someone. It normally would help your teammate reach his/her next lvl and also give you back some XP, but sometimes isntead of that you can only see the animation start and at the end Zilean just gives himself movement speed with his E and nothing happens. At first I thought that I'm just clumsy and pressed E on myself but it kept occuring... I will try to post a video next time it happens.