Spurious min-free-space-percent error

Following the GNOME BuildStream tutorial, I've been hitting the following build error:

GLib.GError: g-io-error-quark: min-free-space-percent '3%' would be exceeded, 5.6 GB available (0)

I've also seen it expressed as:

Failed to fetch ref 'debian/testing/amd64' from 'origin': min-free-space-percent '3%' would be exceeded, 5.3 GB available

The partition that I'm using has 15GB (7.8%) free, so I'm not sure why I'm getting the error. Setting min-free-space-percent to 0 in ~/.cache/buildstream/artifacts/ostree/config doesn't have any effect.

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