diff --git a/src/api/client/sync.rs b/src/api/client/sync.rs
index 9b0facc0..544fe8b6 100644
--- a/src/api/client/sync.rs
+++ b/src/api/client/sync.rs
@@ -7,12 +7,13 @@ use std::{
 
 use axum::extract::State;
 use conduit::{
-	error,
+	debug, error,
 	utils::math::{ruma_from_u64, ruma_from_usize, usize_from_ruma, usize_from_u64_truncated},
 	warn, Err, PduCount,
 };
 use ruma::{
 	api::client::{
+		error::ErrorKind,
 		filter::{FilterDefinition, LazyLoadOptions},
 		sync::sync_events::{
 			self,
@@ -1081,7 +1082,7 @@ fn share_encrypted_room(
 /// Sliding Sync endpoint (future endpoint: `/_matrix/client/v4/sync`)
 pub(crate) async fn sync_events_v4_route(
 	State(services): State<crate::State>, body: Ruma<sync_events::v4::Request>,
-) -> Result<sync_events::v4::Response, RumaResponse<UiaaResponse>> {
+) -> Result<sync_events::v4::Response> {
 	let sender_user = body.sender_user.expect("user is authenticated");
 	let sender_device = body.sender_device.expect("user is authenticated");
 	let mut body = body.body;
@@ -1101,6 +1102,19 @@ pub(crate) async fn sync_events_v4_route(
 		.and_then(|string| string.parse().ok())
 		.unwrap_or(0);
 
+	if globalsince != 0
+		&& !services
+			.users
+			.remembered(sender_user.clone(), sender_device.clone(), conn_id.clone())
+	{
+		debug!("Restarting sync stream because it was gone from the database");
+		return Err(Error::Request(
+			ErrorKind::UnknownPos,
+			"Connection data lost since last time".into(),
+			http::StatusCode::BAD_REQUEST,
+		));
+	}
+
 	if globalsince == 0 {
 		services
 			.users
diff --git a/src/service/users/mod.rs b/src/service/users/mod.rs
index bdb07310..35f30a61 100644
--- a/src/service/users/mod.rs
+++ b/src/service/users/mod.rs
@@ -68,6 +68,13 @@ impl Service {
 	#[inline]
 	pub fn exists(&self, user_id: &UserId) -> Result<bool> { self.db.exists(user_id) }
 
+	pub fn remembered(&self, user_id: OwnedUserId, device_id: OwnedDeviceId, conn_id: String) -> bool {
+		self.connections
+			.lock()
+			.unwrap()
+			.contains_key(&(user_id, device_id, conn_id))
+	}
+
 	pub fn forget_sync_request_connection(&self, user_id: OwnedUserId, device_id: OwnedDeviceId, conn_id: String) {
 		self.connections
 			.lock()