Problem with json-bin feature

[DavePearce]

There is a problem with the json-bin feature relating to interleaved constraints. Here is the minimal example:

(defcolumns X Y)
(definterleaved Z (X Y))

Compiling this with the following command-line:

cargo run --features json-bin -- compile --json --out test.bin test.lisp

Gives the following failure:

Error: key must be a string

With the following backtrace:

   0: anyhow::error::<impl core::convert::From<E> for anyhow::Error>::from
             at /home/djp/.cargo/registry/src/index.crates.io-6f17d22bba15001f/anyhow-1.0.79/src/error.rs:565:25
   1: <core::result::Result<T,F> as core::ops::try_trait::FromResidual<core::result::Result<core::convert::Infallible,E>>>::from_residual
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/core/src/result.rs:1963:27
....