Failing to find bootloader dependency when it's conditionally included

I'm writing a kernel that runs on both the Raspberry Pi 3 and x86_64. I'm trying to use bootloader+bootimage exclusively for the x86_64 version, but not any other architecture. As a result, my Cargo.toml looks like this:

[dependencies]
# common, architecture-independent dependencies

[target.'cfg(target_arch = "x86_64")'.dependencies]
bootloader = "^0.9.8"

When I compile this project with cargo bootimage --target targets/x86_64-unknown-none-softfloat.json --release I get this error message:

Error: An error occured while trying to build the bootloader: Could not find required key `packages[name = `bootloader`` in cargo metadata output

However, if I change the bootloader dependency to not be conditional, i.e. if I just put it in the [dependencies] section, the above error goes away and the project boots perfectly.

Is there some extra configuration I need to do to make this work? It would be nice for me to be able to completely exclude the bootloader dependency off of x86.

Reproduce the failure using the Cargo.toml target-specific dependency and cargo bootimage --target targets/x86_64-unknown-none-softfloat.json --release. Start by tracing how the bootimage command obtains Cargo metadata and locates the bootloader package. Done means the conditional bootloader dependency is found for x86_64 and the image builds without requiring it for other architectures.