Old libidn version, cp target Not a directory

sratoolkit needs libidn.so.11 this version was introduced to nixpkgs at (libidn: 1.33 -> 1.34 by r-ryantm · Pull Request #38290 · NixOS/nixpkgs · GitHub), and was then later overwritten by a newer version. The old code no longer builds. The error cp: target 'po/zh_CN.po': Not a directory is confusing because the zh_CN.po is a file, and not a directory, but why would that result in an error?

patching sources
configuring
fixing libtool script ./build-aux/ltmain.sh
no configure script, doing nothing
building
build flags: SHELL=/nix/store/dd3713mm8wql4r2d5jxx0f58g16nfm4h-bash-5.1-p16/bin/bash
for f in po/*.po.in; do \
        cp $f `echo $f | sed 's/.in//'`; \
done
cp: target 'po/zh_CN.po': Not a directory

I have checked out 47e497b6 (libidn: 1.33 → 1.34, from 2018), and cherry-picked fa6064ad (top-level: fix nix-shell eval w/nixUnstable) and executed nix-shell -p libidn, results in the same error. Should Nix not be able to build this old version?

for f in po/*.po.in; do \
        cp $f `echo $f | sed 's/.in//'`; \
done
cp: target 'po/zh_CN.po' is not a directory

The issue is not reproducible on another machine.