"Could not read the output of 'nix-prefetch-url'. This is a bug."

When trying out Niv and following the example from the readme to add GHC:

$ niv add ghc   \
    -v 8.4.3    \
    -t 'https://downloads.haskell.org/~ghc/<version>/ghc-<version>-i386-deb8-linux.tar.xz'

But instead using the latest ghc version (9.0.1 as of writing), I get the following error:

$ niv add ghc -v 9.0.1 -t 'https://downloads.haskell.org/~ghc/<version>/ghc-<version>-i386-deb8-linux.tar.xz'
Adding package ghc
  FATAL: Could not read the output of 'nix-prefetch-url'. This is a bug. Please create a
  ticket:
  
    https://github.com/nmattia/niv/issues/new
  
  Thanks! I'll buy you a beer.
  command: 
  nix-prefetch-urlhttps://downloads.haskell.org/~ghc/9.0.1/ghc-9.0.1-i386-deb8-linux.tar.xz --name ghc-9.0.1-i386-deb8-linux.tar.xz
  stdout: 
  
  stderr: 
  error: unable to download 'https://downloads.haskell.org/~ghc/9.0.1/ghc-9.0.1-i386-deb8-linux.tar.xz': HTTP error 404
  
  FATAL: One or more packages failed to update:
  ghc: ExitFailure 1
ERROR: ExitFailure 1

The solution is quite simple, simply change deb8 to deb9, as no deb8 file exists. However, the error asked me to post this so I did.

The command that does work:

niv add ghc -v 9.0.1 -t 'https://downloads.haskell.org/~ghc/<version>/ghc-<version>-i386-deb9-linux.tar.xz'

I got the same problem when I tried version 8.10.6, for the same reason and with same fix.