diff --git a/.github/workflows/test-package.yml b/.github/workflows/test-package.yml
index 816038e1..1c3721ab 100644
--- a/.github/workflows/test-package.yml
+++ b/.github/workflows/test-package.yml
@@ -54,6 +54,8 @@ jobs:
         python -m pip install --upgrade pip
         pip install -r requirements-dev.txt
     - name: Test install with pytest (including doctests)
+      env:
+        PYTHONWARNINGS: error
       run: |
         python -c "import tests"  # Compiles hissp.basic on package import.
         pytest -v --cov=hissp --cov-report=xml --doctest-modules --doctest-glob *.md src/hissp/*.lissp tests/ docs/ $(python -c "import hissp; print(hissp.__path__[0])")