diff --git a/src/allmydata/test/common_system.py b/src/allmydata/test/common_system.py
index cfb6c9f04..83ae87883 100644
--- a/src/allmydata/test/common_system.py
+++ b/src/allmydata/test/common_system.py
@@ -685,6 +685,10 @@ class SystemTestMixin(pollmixin.PollMixin, testutil.StallMixin):
     REDUCE_HTTP_CLIENT_TIMEOUT : bool = True
 
     def setUp(self):
+        if os.getenv("TAHOE_DEBUG_BLOCKING") == "1":
+            from .blocking import catch_blocking_in_event_loop
+            catch_blocking_in_event_loop(self)
+
         self._http_client_pools = []
         http_client.StorageClientFactory.start_test_mode(self._got_new_http_connection_pool)
         self.addCleanup(http_client.StorageClientFactory.stop_test_mode)