Division by zero error when plotting a gradient

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3 months ago by
What I want to do is plot \gradient\lnorm\nabla u\rnorm_2^2, where u is the solution to my PDE:

solve(a == L, u, bc)
v = dot(grad(u), grad(u))
plot(grad(v))

When I run this code, I get:

/usr/lib/python2.7/dist-packages/matplotlib/quiver.py:645: RuntimeWarning: divide by zero encountered in double_scalars
length = a * (widthu_per_lenu / (self.scale * self.width))
/usr/lib/python2.7/dist-packages/matplotlib/quiver.py:645: RuntimeWarning: invalid value encountered in multiply
length = a * (widthu_per_lenu / (self.scale * self.width))

The problem is with plotting, and not computing the gradient. Just calling grad(u) on its own causes no error.
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