diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 997bda9bfc0d7136a328cf30e4bbdf3812ae679a..bbd14724b570a02d027a098906229d434504200a 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -69,7 +69,7 @@ unit-tests:
     - |
       set -x
       # GRID is set in one of the "instantiations" of this job template.
-      nix-shell --run "morph build --show-trace morph/grid/${GRID}/grid.nix"
+      nix-shell --pure --run "morph build --show-trace morph/grid/${GRID}/grid.nix"
 
 
 morph-build-localdev: