The indeterminacy locus of the Voisin map

Beauville and Donagi proved that the variety of lines $F(Y)$ of a smooth cubic fourfold $Y$ is a hyperk"ahler variety. Recently, C. Lehn, M.Lehn, Sorger and van Straten proved that one can naturally associate a hyperK"ahler variety $Z(Y)$ to the variety of twisted cubics on $Y$. Then, Voisin defined a degree 6 rational map $\psi :F(Y)\times F(Y)⇢Z(Y)$. We will show that the indeterminacy locus of $\psi$ is the locus of intersecting lines.