bug#49862: While downloading substitutes: Wrong type argument in position 1 (expecting struct): #f

[Roy Pan]

Hello, Everyone:

I have the same problem during i exec
```
> guix pull --bootstrap   --no-substitutes
```

```
Starting download of
/gnu/store/2jbnapabhps7icj5gniksji4r4j47apm-mkfontscale-1.2.3.tar.xz
From
https://ci.guix.gnu.org/file/mkfontscale-1.2.3.tar.xz/sha256/0pp7dyfrrkrqxslk9q8660k0h4swaqlixsnnph2fxb7i8k1ws899.
..
In procedure struct-vtable: Wrong type argument in position 1 (expecting
struct): 145584
```

Thanks,
Roy