Robustfit gives output equals as zero depending on the order of the variables

Hello! This is my first question here. First, I'm really thankful that I have found answer to almost any of my questions in the forums.

My problem now is that I'm using the robustfit function, but when I write robustfit(x,y) sometimes it won't find any fit and the output is just 0. It works when I use robustfit(y,x), but I don't find any reason why it shouldn't work the other way around. Both of my variables are 1 dimensional columns.

Thank you!