  Consider a $2n$-dimensional symplectic vector space $E$ over an arbitrary
field $\mathbb{F}$. Given a contraction map $f: \wedge^n E \rightarrow
\wedge^{n-2} E$ such that the Lagrangian--Grassmannian
$L(n,2n)=G(n,2n)\cap{\mathbb P}(\ker f)$, where $\wedge^r E$ denotes the $r$-th
exterior power of $E$ and ${\mathbb P}(\ker f)$ is the projectivization of
$\ker f$. In this paper, for a symplectic vector space $E$ of dimension $n=6$,
we prove that the surjectivity of the contraction map $f:\wedge^{6} E
\rightarrow \wedge^{4} E$ depends on the characteristic of the base field and
we calculate the codimension of the linear section ${\mathbb P}(\ker
f)\subseteq {\mathbb P}(\wedge^{6}E)$ for any characteristic.
