diff --git a/scrapy/downloadermiddlewares/offsite.py b/scrapy/downloadermiddlewares/offsite.py
index dca92631f..ea2c2f030 100644
--- a/scrapy/downloadermiddlewares/offsite.py
+++ b/scrapy/downloadermiddlewares/offsite.py
@@ -131,9 +131,7 @@ class OffsiteMiddleware:
             return re.compile("")  # allow all by default
 
         domains = self._process_domains(allowed_domains, "allowed_domains")
-        if domains:
-            return re.compile(rf"^(.*\.)?({'|'.join(domains)})$")
-        return re.compile("")  # allow all if no valid domains remain
+        return re.compile(rf"^(.*\.)?({'|'.join(domains)})$")
 
     def _get_disallowed_host_regex(self, spider: Spider) -> re.Pattern[str] | None:
         """Build a regex that positively matches disallowed hosts.
@@ -146,6 +144,4 @@ class OffsiteMiddleware:
             return None
 
         domains = self._process_domains(disallowed_domains, "disallowed_domains")
-        if domains:
-            return re.compile(rf"^(.*\.)?({'|'.join(domains)})$")
-        return None
+        return re.compile(rf"^(.*\.)?({'|'.join(domains)})$")
diff --git a/tests/test_downloadermiddleware_offsite.py b/tests/test_downloadermiddleware_offsite.py
index cfa8f146a..1d459c8cb 100644
--- a/tests/test_downloadermiddleware_offsite.py
+++ b/tests/test_downloadermiddleware_offsite.py
@@ -1,4 +1,5 @@
 import logging
+from unittest.mock import AsyncMock, patch
 
 import pytest
 
@@ -125,6 +126,25 @@ def test_process_request_invalid_domains(caplog):
     assert "Invalid domain configuration" in caplog.text
 
 
+def test_invalid_domains_closes_spider(caplog):
+    crawler = get_crawler(Spider)
+    crawler.spider = crawler._create_spider(
+        name="a", allowed_domains=["a.example", None]
+    )
+    mw = OffsiteMiddleware.from_crawler(crawler)
+    mock_engine = AsyncMock()
+    crawler.engine = mock_engine
+    with (
+        patch(
+            "scrapy.downloadermiddlewares.offsite._schedule_coro"
+        ) as mock_schedule,
+        caplog.at_level(logging.ERROR),
+    ):
+        mw.spider_opened(crawler.spider)
+    assert "Invalid domain configuration" in caplog.text
+    mock_schedule.assert_called_once()
+
+
 @pytest.mark.parametrize(
     ("allowed_domain", "url", "allowed"),
     [