Getting a function error "Output argument "z" (and maybe others) not assigned during call"

Hey guys im new to matlab ive been trying to do a power function but I've been getting this error and Im not understanding why

Here is the function

 function [z] = power1(x,y)
  if y>=0
    if y==0
        z=1;
    end
    if y==1
        z=x;
    end
    z=x*power1(x,y-1);
end
end

Answer 1

[Alexandra Harkai]

If y<0, variable z doesn't get any value assigned to it.

See the same question here here.