Why does copying closure over `ssh-ng://` fail where `ssh://` succeeds?

I have run into this strange error when rebuilding system and copying closure to remote system: # nixos-rebuild switch --log-format internal-json --verbose --flake .#remotetarget1 --target-host "ssh-ng://remotetarget1" |& nom --json ... copying 39 paths copying path '/nix/store/1bbq14qfhiglhm8i5dpw5ny3awwkn1b0-system-generators' to 'ssh-ng://remotetarget1' ... error: cannot add path '/nix/store/1bbq14qfhiglhm8i5dpw5ny3awwkn1b0-system-generators' because it lacks a signature by a trusted key ......