  We calculate the electrically induced spin accumulation in diffusive systems
due to both Rashba (with strength $\alpha)$ and Dresselhaus (with strength
$\beta)$ spin-orbit interaction. Using a diffusion equation approach we find
that magnetoelectric effects disappear and that there is thus no spin
accumulation when both interactions have the same strength, $\alpha=\pm \beta$.
In thermodynamically large systems, the finite spin accumulation predicted by
Chaplik, Entin and Magarill, [Physica E {\bf 13}, 744 (2002)] and by Trushin
and Schliemann [Phys. Rev. B {\bf 75}, 155323 (2007)] is recovered an
infinitesimally small distance away from the singular point $\alpha=\pm \beta$.
We show however that the singularity is broadened and that the suppression of
spin accumulation becomes physically relevant (i) in finite-sized systems of
size $L$, (ii) in the presence of a cubic Dresselhaus interaction of strength
$\gamma$, or (iii) for finite frequency measurements. We obtain the parametric
range over which the magnetoelectric effect is suppressed in these three
instances as (i) $|\alpha|-|\beta| \lesssim 1/mL$, (ii)$|\alpha|-|\beta|
\lesssim \gamma p_{\rm F}^2$, and (iii) $|\alpha|-|\beta| \lesssiM
\sqrt{\omega/m p_{\rm F}\ell}$ with $\ell$ the elastic mean free path and
$p_{\rm F}$ the Fermi momentum. We attribute the absence of spin accumulation
close to $\alpha=\pm \beta$ to the underlying U (1) symmetry. We illustrate and
confirm our predictions numerically.
