diff --git a/pytest_mergify/ci_insights.py b/pytest_mergify/ci_insights.py
index 9a3bd31..f0d5649 100644
--- a/pytest_mergify/ci_insights.py
+++ b/pytest_mergify/ci_insights.py
@@ -151,11 +151,12 @@ def __post_init__(self) -> None:
         self.tracer_provider.add_span_processor(span_processor)
         self.tracer = self.tracer_provider.get_tracer("pytest-mergify")
 
-        # Retrieve the branch name, preferring base ref (target branch) over head ref.
+        # Retrieve the branch name, preferring base ref (target branch) over
+        # head ref. `or` ensures an empty base ref (e.g. `GITHUB_BASE_REF=""` on
+        # push runs) falls through to the head ref.
         branch_name = resource.attributes.get(
-            vcs_attributes.VCS_REF_BASE_NAME,
-            resource.attributes.get(vcs_attributes.VCS_REF_HEAD_NAME),
-        )
+            vcs_attributes.VCS_REF_BASE_NAME
+        ) or resource.attributes.get(vcs_attributes.VCS_REF_HEAD_NAME)
         if branch_name is not None:
             # `str` cast just for `mypy`.
             self.branch_name = str(branch_name)
diff --git a/tests/test_ci_insights.py b/tests/test_ci_insights.py
index 2ea232f..6932b5a 100644
--- a/tests/test_ci_insights.py
+++ b/tests/test_ci_insights.py
@@ -685,3 +685,35 @@ def test_bar():
         plugin.mergify_ci.flaky_detector._available_budget_duration.total_seconds()
         == datetime.timedelta(seconds=5).total_seconds()
     )
+
+
+@responses.activate
+def test_empty_base_ref_falls_through_to_head_ref(
+    monkeypatch: pytest.MonkeyPatch,
+    pytester: _pytest.pytester.Pytester,
+) -> None:
+    """When `GITHUB_BASE_REF` is empty (push/scheduled runs), `branch_name`
+    should fall through to the HEAD ref so quarantine still works."""
+    _set_test_environment(monkeypatch, mode="unhealthy")
+
+    # Set an empty `GITHUB_BASE_REF` to simulate push runs where the env var
+    # exists but is empty.
+    monkeypatch.setenv("GITHUB_BASE_REF", "")
+
+    _make_quarantine_mock()
+    _make_flaky_detection_context_mock()
+
+    pytester.makepyfile(
+        """
+        def test_foo():
+            assert True
+        """
+    )
+
+    plugin = pytest_mergify.PytestMergify()
+    pytester.runpytest_inprocess(plugins=[plugin])
+
+    # `branch_name` should come from HEAD ref, not the empty base ref.
+    assert plugin.mergify_ci.branch_name == "main"
+    assert plugin.mergify_ci.flaky_detector is not None
+    assert plugin.mergify_ci.flaky_detector.mode == "unhealthy"