### Numerical integration and plot failing

I have a function which can be efficiently calculated on some interval on the real line
and if I feed sage with concrete arguments, sage gives concrete numerical values.
Nevertheless, if I try to integrate it numerically (or plot it), something goes terribly wrong
and sage is not able to calculate the values.

In my case I get the error message
"negative number cannot be raised to a fractional power"

The function itself is a solution to the cubic equation so it involves square root and a cubic root.

is it some bug in sage and if there is some workaround?

MINIMAL NON_WORKING EXAMPLE

y = var('y');
z = var('z');

cauchy= y.substitute(solve(z*y^3 +y^2 - 2*z*y+2 ,y)[0])

def F(u):
return arg(cauchy).substitute(z=u).n()

numerical_integral(F,1,2)