diff --git a/cachecontrol/heuristics.py b/cachecontrol/heuristics.py
index d95e78a9..3d82d360 100644
--- a/cachecontrol/heuristics.py
+++ b/cachecontrol/heuristics.py
@@ -137,7 +137,8 @@ def update_headers(self, resp: HTTPResponse) -> dict[str, str]:
             return {}
 
         time_tuple = parsedate_tz(headers["date"])
-        assert time_tuple is not None
+        if time_tuple is None:
+            return {}
         date = calendar.timegm(time_tuple[:6])
         last_modified = parsedate(headers["last-modified"])
         if last_modified is None:
diff --git a/tests/test_expires_heuristics.py b/tests/test_expires_heuristics.py
index 2a5fd23b..491a1400 100644
--- a/tests/test_expires_heuristics.py
+++ b/tests/test_expires_heuristics.py
@@ -9,6 +9,7 @@
 from pprint import pprint
 from unittest.mock import Mock
 
+import pytest
 from requests import Session, get
 
 from cachecontrol import CacheControl
@@ -153,6 +154,14 @@ def test_expires_is_not_replaced_when_present(self):
         resp = DummyResponse(200, {"Expires": self.day_ahead})
         assert self.heuristic.update_headers(resp) == {}
 
+    @pytest.mark.parametrize("date", ["", "not-a-date"])
+    def test_invalid_date_does_not_infer_expiry(self, date):
+        headers = {"Date": date, "Last-Modified": self.week_ago}
+        resp = DummyResponse(200, headers)
+        assert self.heuristic.update_headers(resp) == {}
+        assert self.heuristic.apply(resp) is resp
+        assert dict(resp.headers) == headers
+
     def test_last_modified_is_used(self):
         resp = DummyResponse(200, {"Date": self.now, "Last-Modified": self.week_ago})
         modified = self.heuristic.update_headers(resp)