diff --git a/zulip_bots/zulip_bots/bots/flock/fixtures/test_found_invalid_recipient.json b/zulip_bots/zulip_bots/bots/flock/fixtures/test_found_invalid_recipient.json
new file mode 100644
index 0000000..85467a0
--- /dev/null
+++ b/zulip_bots/zulip_bots/bots/flock/fixtures/test_found_invalid_recipient.json
@@ -0,0 +1,18 @@
+{
+    "request": {
+        "api_url": "https://api.flock.co/v1/roster.listContacts",
+        "method": "GET",
+        "params": {
+            "token": "12345"
+        }
+    },
+    "response": [
+        {
+            "id": 1234567890123456789012345678901,
+            "firstName": "david"
+        }
+    ],
+    "response-headers": {
+        "content-type": "application/json; charset=utf-8"
+    }
+}
diff --git a/zulip_bots/zulip_bots/bots/flock/flock.py b/zulip_bots/zulip_bots/bots/flock/flock.py
index 5249c33..d6eb660 100644
--- a/zulip_bots/zulip_bots/bots/flock/flock.py
+++ b/zulip_bots/zulip_bots/bots/flock/flock.py
@@ -62,7 +62,7 @@ def get_flock_response(content: str, config: Dict[str, str]) -> str:
         return error
 
     if len(str(recipient_id)) > 30:
-        return recipient_id
+        return "Found user is invalid."
 
     payload = {
         'to': recipient_id,
diff --git a/zulip_bots/zulip_bots/bots/flock/test_flock.py b/zulip_bots/zulip_bots/bots/flock/test_flock.py
index f1cb4b2..251cc23 100644
--- a/zulip_bots/zulip_bots/bots/flock/test_flock.py
+++ b/zulip_bots/zulip_bots/bots/flock/test_flock.py
@@ -43,6 +43,12 @@ right now.\nPlease try again later")
                 self.mock_http_conversation('test_no_recipient_found'):
                     self.verify_reply('david: hello', bot_response)
 
+    def test_found_invalid_recipient(self) -> None:
+        bot_response = "Found user is invalid."
+        with self.mock_config_info(self.normal_config), \
+                self.mock_http_conversation('test_found_invalid_recipient'):
+                    self.verify_reply('david: hello', bot_response)
+
     @patch('zulip_bots.bots.flock.flock.get_recipient_id')
     def test_message_send_success(self, get_recipient_id: str) -> None:
         bot_response = "Message sent."