diff --git a/ci-tools/update-nixpkgs b/ci-tools/update-nixpkgs
index 6b4438ac259671ba0278d4e4f9c5cb129e8d6306..9267393f9e14fb0fa3d7f2965dc02fffdcd6fd61 100755
--- a/ci-tools/update-nixpkgs
+++ b/ci-tools/update-nixpkgs
@@ -59,7 +59,7 @@ nix-build -A morph -o result-before
 
 # Spawn *another* nix-shell that has the *other* update-nixpkgs tool.  Should
 # sort out this mess sooner rather than later...
-nix-shell ../shell.nix --run 'update-nixpkgs ./nixpkgs.json'
+nix-shell ../shell.nix --run 'update-nixpkgs ${PWD}/nixpkgs.json'
 
 # Show us what we did
 if git diff --exit-code; then