decimal::d128 with alga::general::real

I suspect this is more of a usage error than a bug, but can't anywhere else to go for help.

I'm trying to use the ncollision2d and decimal crates, and keep getting this error:
```
error[E0277]: the trait bound `decimal::d128: alga::general::Real` is not satisfied
--> src/misc.rs:86:3
|
86 | pub cuboid: Cuboid,
| ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `alga::general::Real` is not implemented for `decimal::d128`
|
= note: required by `ncollide2d::shape::Cuboid`
```
Among others, I have this in my Cargo.toml:
```
[dependencies]
ncollide2d = "0.17.1"
decimal = "2.0.4"

[dependencies.alga]
version = "0.7.1"
features = ["decimal"]
```
As far as I can tell, that should enable the Real trait for decimal, but it isn't working. Is this a problem with alga? If not, can you tell me what I'm missing?

Thanks.