diff --git a/tests/conftest.py b/tests/conftest.py
index 6930599a7..5b865952f 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -32,7 +32,9 @@ _API_KEY_ENV_VARS = (
 @pytest.fixture(autouse=True)
 def _dummy_api_keys(monkeypatch):
     for env_var in _API_KEY_ENV_VARS:
-        monkeypatch.setenv(env_var, os.environ.get(env_var, "placeholder"))
+        # `or` not a .get default: an env var present but empty (e.g. a key left
+        # blank in a .env copied from .env.example) must still get the placeholder.
+        monkeypatch.setenv(env_var, os.environ.get(env_var) or "placeholder")
 
 
 @pytest.fixture(autouse=True)