  For $d\geq 3$ we give an example of a constant coefficient surjective
differential operator $P(D):\mathscr{D}'(X)\rightarrow\mathscr{D}'(X)$ over
some open subset $X\subset\R^d$ such that
$P^+(D):\mathscr{D}'(X\times\R)\rightarrow\mathscr{D}'(X\times\R)$ is not
surjective, where $P^+(x_1,...,x_{d+1}):=P(x_1,...,x_d)$. This answers in the
negative a problem posed by Bonet and Doma\'nski in \cite[Problem 9.1]{Bonet}.
