### abs and plot problem

Hi there.
Using Cocalc, I am encountering problems trying to plot some functions.
The following simple example shows how I am able to plot the modulus of a simple function using my own modulus function, but how it fails using abs(). Which is really weird.

_ = var('a,x')

MODULUS(x) = sqrt(real(x)^2 + imag(x)^2)

Hmod = MODULUS(a*x + i*x^2)

Habs = abs(a*x + i*x^2)

plot([Hmod(a=n) for n in [1..5]], (x,0, 2))

plot([Habs(a=n) for n in [1..5]], (x,0, 2))

Hmod can be plotted, while Habs will return "TypeError: unable to coerce to a real number".

On the other hand the following works:

plot(Habs(a=1), (x,0, 2))

So it works without list comprehension and with my MODULUS function... Don't know what to think of that.

Anyone can help on that? I have seriously more complicated cases that I seem to be unable to plot because of this very problem.
Thanks a lot.