Taking the dual of an elliptic curve isogeny raises AssertionError

[pjbruin]

Steps To Reproduce

We create an isogeny between elliptic curves over a finite field of 23^2 elements:

sage: R.<x> = GF(23)[]
sage: F.<a> = FiniteField(23^2, modulus=x^2-x+1)
sage: E0 = EllipticCurve(F, (0, 1))
sage: E1 = EllipticCurve(F, (8, 1))
sage: phi = E0.isogeny(kernel=E0((a, 0)), codomain=E1)
sage: phi.dual()

Expected Behavior

The dual of phi should be returned.

Actual Behavior

The following error is raised:

Traceback (most recent call last):
...
AssertionError: bug in dual()

Additional Information

The error is not raised when codomain=E1 is omitted.

Environment

- **Sage Version**: SageMath 10.3.beta6

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide

[pjbruin]

@yyyyx4 the Git log says you wrote the relevant part of dual(), could you have a look?

[yyyyx4]

Interesting, thanks for reporting this. I'll have a look approximately tomorrow.