### Discriminant of multivariate polynomials with complex coefficients

I would like to compute the discriminant of some polynomial in two variables, x and z, the discriminant should be computed with respect to x.
I am able to do so in certain cases, but not when the coefficients are complex numbers written in a certain way.
For example, this works:

```
R.<z> = QQ[]
S.<x> = R[]
p = x**2 - z**2
p.discriminant()
```

gives:

```
4*z^2
```

But when I try to use a complex coefficient, discriminant fails because the expression is regarded as a symbolic expression, instead of a polynomial

```
q = x**2 - z**2 * I
q.discriminant()
AttributeError: 'sage.symbolic.expression.Expression' object has no attribute 'discriminant'
```

Curiously, if I use instead

```
q = x**2 - z**2 * 1j
q.discriminant()
```

this gives the correct answer.
Now for particular reasons I really need to deal with 'I' as the complex unit, and cannot use 'j'. What's wrong with 'I'?