  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 $\psi:F(Y)\times F(Y)\dashrightarrow Z(Y)$. We will show
that the indeterminacy locus of $\psi$ is the locus of intersecting lines.
