Paragraph ended before split was complete?

I have the following

begin{equation}
    begin{split}
        frac{1}{2}sum_{i} z_i^2 + sum_{i}s_it_i &=   frac{1}{2}sum_{i}z_i^2 + sum_{P_2}s_it_i+sum_{R setminus P_2}s_it_i
                             &=     frac{1}{2}sum_i z_i^2 + sum_{P_2}2(n-3-z_i) + sum_{R setminus P_2} (n-2-z_i)
                             &ge    frac{1}{2}sum_i z_i^2 - sum_{R} z_i - sum_{P_2}z_i+ 2(n-3)sigma + (n-p-q-2)(n-2)

    end{split}
end{equation}

Compiling gives me the error "Paragraph ended before split was complete", it mentions something about par afterward. I can’t spot any error.