  Consider a totally irregular measure $\mu$ in $\mathbb{R}^{n+1}$, that is,
the upper density $\limsup_{r\to0}\frac{\mu(B(x,r))}{(2r)^n}$ is positive
$\mu$-a.e.\ in $\mathbb{R}^{n+1}$, and the lower density
$\liminf_{r\to0}\frac{\mu(B(x,r))}{(2r)^n}$ vanishes $\mu$-a.e. in
$\mathbb{R}^{n+1}$. We show that if $T_\mu f(x)=\int K(x,y)\,d\mu(y)$ is an
operator whose kernel $K(\cdot,\cdot)$ is the gradient of the fundamental
solution for a uniformly elliptic operator in divergence form associated with a
matrix with H\"older continuous coefficients, then $T_\mu$ is not bounded in
$L^2(\mu)$. This extends a celebrated result proved previously by Eiderman,
Nazarov and Volberg for the $n$-dimensional Riesz transform.
