diff --git a/synapse/http/matrixfederationclient.py b/synapse/http/matrixfederationclient.py
index b27c4c1c3..3c27686a8 100644
--- a/synapse/http/matrixfederationclient.py
+++ b/synapse/http/matrixfederationclient.py
@@ -221,30 +221,31 @@ class MatrixFederationHttpClient(object):
         """
         try:
             response = yield self._send_request(**send_request_args)
-        except HttpResponseException as e:
-            # Received a 400. Raise unless we're retrying
-            if not try_trailing_slash_on_400:
-                raise e
 
-        # Check if it's necessary to retry with a trailing slash
-        body = yield _handle_json_response(
-            self.hs.get_reactor(), self.default_timeout, request, response,
-        )
-
-        # Retry with a trailing slash if we received a 400 with
-        # 'M_UNRECOGNIZED' which some endpoints can return when omitting a
-        # trailing slash on Synapse <=v0.99.2.
-        if not (response.code == 400 and body.get("errcode") == "M_UNRECOGNIZED"):
-            # Enable backoff if initially disabled
-            send_request_args["backoff_on_404"] = backoff_on_404
-
-            # Add trailing slash
-            send_request_args["request"].path += "/"
-
-            response = yield self._send_request(**send_request_args)
+            # Check if it's necessary to retry with a trailing slash
             body = yield _handle_json_response(
                 self.hs.get_reactor(), self.default_timeout, request, response,
             )
+        except HttpResponseException as e:
+            if not try_trailing_slash_on_400:
+                # Received an error >= 300. Raise unless we're retrying
+                raise e
+        except:
+            raise e
+
+        # Retry with a trailing slash if we received a 400 with
+        # 'M_UNRECOGNIZED' which some endpoints can return when omitting a
+        # trailing slash on Synapse <= v0.99.2.
+        # Enable backoff if initially disabled
+        send_request_args["backoff_on_404"] = backoff_on_404
+
+        # Add trailing slash
+        send_request_args["request"].path += "/"
+
+        response = yield self._send_request(**send_request_args)
+        body = yield _handle_json_response(
+            self.hs.get_reactor(), self.default_timeout, request, response,
+        )
 
         defer.returnValue(body)
 
diff --git a/tests/http/test_fedclient.py b/tests/http/test_fedclient.py
index b45eee3a8..84216db44 100644
--- a/tests/http/test_fedclient.py
+++ b/tests/http/test_fedclient.py
@@ -272,7 +272,7 @@ class FederationClientTests(HomeserverTestCase):
         """
         If a connection is made to a client but the client rejects it due to
         requiring a trailing slash. We need to retry the request with a
-        trailing slash. Workaround for Synapse <=v0.99.2, explained in #3622.
+        trailing slash. Workaround for Synapse <= v0.99.2, explained in #3622.
         """
         d = self.cl.get_json(
             "testserv:8008", "foo/bar", try_trailing_slash_on_400=True,
@@ -302,9 +302,6 @@ class FederationClientTests(HomeserverTestCase):
             b'{"errcode":"M_UNRECOGNIZED","error":"Unrecognized request"}'
         )
 
-        # We should get a 400 response, then try again
-        self.pump()
-
         # We should get another request wiht a trailing slash
         self.assertRegex(conn.value(), b"^GET /foo/bar/")