diff --git a/src/allmydata/storage/http_client.py b/src/allmydata/storage/http_client.py
index cc26d4b37..fed66bb75 100644
--- a/src/allmydata/storage/http_client.py
+++ b/src/allmydata/storage/http_client.py
@@ -297,6 +297,11 @@ class StorageClient(object):
         """
         cls.TEST_MODE_REGISTER_HTTP_POOL = callback
 
+    @classmethod
+    def stop_test_mode(cls):
+        """Stop testing mode."""
+        cls.TEST_MODE_REGISTER_HTTP_POOL = None
+
     # The URL is a HTTPS URL ("https://...").  To construct from a NURL, use
     # ``StorageClient.from_nurl()``.
     _base_url: DecodedURL
diff --git a/src/allmydata/test/common_system.py b/src/allmydata/test/common_system.py
index 90990a8ca..af86440cc 100644
--- a/src/allmydata/test/common_system.py
+++ b/src/allmydata/test/common_system.py
@@ -649,6 +649,7 @@ class SystemTestMixin(pollmixin.PollMixin, testutil.StallMixin):
     def setUp(self):
         self._http_client_pools = []
         http_client.StorageClient.start_test_mode(self._http_client_pools.append)
+        self.addCleanup(http_client.StorageClient.stop_test_mode)
         self.port_assigner = SameProcessStreamEndpointAssigner()
         self.port_assigner.setUp()
         self.addCleanup(self.port_assigner.tearDown)
diff --git a/src/allmydata/test/test_storage_http.py b/src/allmydata/test/test_storage_http.py
index 25c21e03f..87a6a2306 100644
--- a/src/allmydata/test/test_storage_http.py
+++ b/src/allmydata/test/test_storage_http.py
@@ -294,6 +294,7 @@ class CustomHTTPServerTests(SyncTestCase):
         StorageClient.start_test_mode(
             lambda pool: self.addCleanup(pool.closeCachedConnections)
         )
+        self.addCleanup(StorageClient.stop_test_mode)
         # Could be a fixture, but will only be used in this test class so not
         # going to bother:
         self._http_server = TestApp()
@@ -382,6 +383,7 @@ class HttpTestFixture(Fixture):
         StorageClient.start_test_mode(
             lambda pool: self.addCleanup(pool.closeCachedConnections)
         )
+        self.addCleanup(StorageClient.stop_test_mode)
         self.clock = Clock()
         self.tempdir = self.useFixture(TempDir())
         # The global Cooperator used by Twisted (a) used by pull producers in