diff --git a/.github/workflows/mamba.yml b/.github/workflows/mamba.yml
index 3c1b9a2..90198b7 100644
--- a/.github/workflows/mamba.yml
+++ b/.github/workflows/mamba.yml
@@ -60,6 +60,7 @@ jobs:
         pipenv install --dev
 
     # TODO Run each test separately?  (See: #145.)
+    # TODO Remove PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=python hack on upgrading python.
     - name: Run tests
       run: |
-        pipenv run mamba ./
+        PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=python pipenv run mamba ./