Failing to find bootloader dependency when it's conditionally included

[bollian]

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.