diff --git a/pymongo/topology_description.py b/pymongo/topology_description.py
index 5af5bffdf..f691036c3 100644
--- a/pymongo/topology_description.py
+++ b/pymongo/topology_description.py
@@ -166,7 +166,7 @@ def updated_topology_description(topology_description, server_description):
             topology_type = _SERVER_TYPE_TO_TOPOLOGY_TYPE[server_type]
 
     if topology_type == TOPOLOGY_TYPE.Sharded:
-        if server_type != SERVER_TYPE.Mongos:
+        if server_type not in (SERVER_TYPE.Mongos, SERVER_TYPE.Unknown):
             sds.pop(address)
 
     elif topology_type == TOPOLOGY_TYPE.ReplicaSetNoPrimary:
diff --git a/test/discovery_and_monitoring/sharded/mongos_disconnect.json b/test/discovery_and_monitoring/sharded/mongos_disconnect.json
new file mode 100644
index 000000000..f3beb558e
--- /dev/null
+++ b/test/discovery_and_monitoring/sharded/mongos_disconnect.json
@@ -0,0 +1,88 @@
+{
+    "description": "Mongos disconnect", 
+    "phases": [
+        {
+            "outcome": {
+                "servers": {
+                    "a:27017": {
+                        "setName": null, 
+                        "type": "Mongos"
+                    }, 
+                    "b:27017": {
+                        "setName": null, 
+                        "type": "Mongos"
+                    }
+                }, 
+                "setName": null, 
+                "topologyType": "Sharded"
+            }, 
+            "responses": [
+                [
+                    "a:27017", 
+                    {
+                        "ismaster": true, 
+                        "msg": "isdbgrid", 
+                        "ok": 1
+                    }
+                ], 
+                [
+                    "b:27017", 
+                    {
+                        "ismaster": true, 
+                        "msg": "isdbgrid", 
+                        "ok": 1
+                    }
+                ]
+            ]
+        }, 
+        {
+            "outcome": {
+                "servers": {
+                    "a:27017": {
+                        "setName": null, 
+                        "type": "Unknown"
+                    }, 
+                    "b:27017": {
+                        "setName": null, 
+                        "type": "Mongos"
+                    }
+                }, 
+                "setName": null, 
+                "topologyType": "Sharded"
+            }, 
+            "responses": [
+                [
+                    "a:27017", 
+                    {}
+                ]
+            ]
+        }, 
+        {
+            "outcome": {
+                "servers": {
+                    "a:27017": {
+                        "setName": null, 
+                        "type": "Mongos"
+                    }, 
+                    "b:27017": {
+                        "setName": null, 
+                        "type": "Mongos"
+                    }
+                }, 
+                "setName": null, 
+                "topologyType": "Sharded"
+            }, 
+            "responses": [
+                [
+                    "a:27017", 
+                    {
+                        "ismaster": true, 
+                        "msg": "isdbgrid", 
+                        "ok": 1
+                    }
+                ]
+            ]
+        }
+    ], 
+    "uri": "mongodb://a,b"
+}