Build failure: "Cache key missing"

When I run bst build core/epiphany.bst today, it fails inexplicably with this error:

    [--:--:--] START   [core/epiphany.bst]: Caching Artifact
    [00:00:00] FAILURE [core/epiphany.bst]: Caching Artifact
    [00:00:38] FAILURE [core/epiphany.bst]: Cache key missing

I've just now reproduced it three times in a row.

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