diff --git a/tests/issue_78_80/__init__.py b/tests/issue_78_80/__init__.py
new file mode 100644
index 00000000..8c089bb4
--- /dev/null
+++ b/tests/issue_78_80/__init__.py
@@ -0,0 +1,6 @@
+"""Keep these fixtures in the issue_78_80 package namespace.
+
+Without this package marker, pytest's prepend import mode loads this directory's
+conftest as the top-level ``conftest`` module. That shadows tests/conftest.py and
+breaks test_service's shared ``installed`` fixture import during full collection.
+"""