Sorting on Attribute with Callable for order Parameter in field

[klausmcm]

I defined a callable for the order parameter and was expecting the third assertion below to pass as well (running on Python 3.7.2)

from decimal import Decimal

from attrs import define, field, validators


@define
class TestClass:
    num: Decimal = field(
        validator=validators.instance_of(Decimal),
        eq=lambda x: 0 if x.is_nan() else x,
        order=lambda x: Decimal("-Infinity") if x.is_nan() else x,
    )


assert sorted([Decimal(1), Decimal("-Infinity")]) == [Decimal("-Infinity"), Decimal(1)]

tc1 = TestClass(num=Decimal(2))
tc2 = TestClass(num=Decimal(0))
assert sorted([tc1, tc2], key=lambda x: x.num) == [tc2, tc1]

tc1 = TestClass(num=Decimal(3))
tc2 = TestClass(num=Decimal("NaN"))
assert sorted([tc1, tc2], key=lambda x: x.num) == [tc2, tc1]

However, I get the following error

Traceback (most recent call last):
  File "test.py", line 23, in <module>
    assert sorted([tc1, tc2], key=lambda x: x.num) == [tc2, tc1]
decimal.InvalidOperation: [<class 'decimal.InvalidOperation'>]

This error is usually raised when trying to sort a collection containing Decimal("NaN") so the callable that I defined for order did not seem to be used when calling sorted. I'm not sure if this is a bug or if I misunderstood something.

[klausmcm]

Sorry, what I'm asking for is what I'm supposed to define as the key parameter in sorted.