diff --git a/changelog.d/7326.misc b/changelog.d/7326.misc
new file mode 100644
index 000000000..676f28537
--- /dev/null
+++ b/changelog.d/7326.misc
@@ -0,0 +1 @@
+Move catchup of replication streams logic to worker.
diff --git a/synapse/replication/tcp/commands.py b/synapse/replication/tcp/commands.py
index 5002efe6a..f26aee83c 100644
--- a/synapse/replication/tcp/commands.py
+++ b/synapse/replication/tcp/commands.py
@@ -462,7 +462,7 @@ def parse_command_from_line(line: str) -> Command:
     Line should already be stripped of whitespace and be checked if blank.
     """
 
-    idx = line.index(" ")
+    idx = line.find(" ")
     if idx >= 0:
         cmd_name = line[:idx]
         rest_of_line = line[idx + 1 :]
diff --git a/tests/replication/tcp/test_commands.py b/tests/replication/tcp/test_commands.py
new file mode 100644
index 000000000..3cbcb513c
--- /dev/null
+++ b/tests/replication/tcp/test_commands.py
@@ -0,0 +1,42 @@
+# -*- coding: utf-8 -*-
+# Copyright 2020 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+from synapse.replication.tcp.commands import (
+    RdataCommand,
+    ReplicateCommand,
+    parse_command_from_line,
+)
+
+from tests.unittest import TestCase
+
+
+class ParseCommandTestCase(TestCase):
+    def test_parse_one_word_command(self):
+        line = "REPLICATE"
+        cmd = parse_command_from_line(line)
+        self.assertIsInstance(cmd, ReplicateCommand)
+
+    def test_parse_rdata(self):
+        line = 'RDATA events 6287863 ["ev", ["$eventid", "!roomid", "type", null, null, null]]'
+        cmd = parse_command_from_line(line)
+        self.assertIsInstance(cmd, RdataCommand)
+        self.assertEqual(cmd.stream_name, "events")
+        self.assertEqual(cmd.token, 6287863)
+
+    def test_parse_rdata_batch(self):
+        line = 'RDATA presence batch ["@foo:example.com", "online"]'
+        cmd = parse_command_from_line(line)
+        self.assertIsInstance(cmd, RdataCommand)
+        self.assertEqual(cmd.stream_name, "presence")
+        self.assertIsNone(cmd.token)