diff --git a/syncapi/streams/stream_invite.go b/syncapi/streams/stream_invite.go
index 700f25c10..542f55722 100644
--- a/syncapi/streams/stream_invite.go
+++ b/syncapi/streams/stream_invite.go
@@ -80,6 +80,7 @@ func (p *InviteStreamProvider) IncrementalSync(
 		if _, ok := req.Response.Rooms.Join[roomID]; ok {
 			continue
 		}
+
 		lr := types.NewLeaveResponse()
 		h := sha256.Sum256(append([]byte(roomID), []byte(strconv.FormatInt(int64(to), 10))...))
 		lr.Timeline.Events = append(lr.Timeline.Events, gomatrixserverlib.ClientEvent{
diff --git a/syncapi/streams/streams.go b/syncapi/streams/streams.go
index caa428827..c3b2ac348 100644
--- a/syncapi/streams/streams.go
+++ b/syncapi/streams/streams.go
@@ -128,7 +128,6 @@ func ToToken(provider StreamProvider, position types.StreamPosition) types.Strea
 	default:
 		panic(fmt.Sprintf("unknown stream provider: %T", t))
 	}
-	return types.StreamingToken{}
 }
 
 func IncrementalPositions(provider StreamProvider, current, since types.StreamingToken) (types.StreamPosition, types.StreamPosition) {
@@ -154,5 +153,4 @@ func IncrementalPositions(provider StreamProvider, current, since types.Streamin
 	default:
 		panic(fmt.Sprintf("unknown stream provider: %T", t))
 	}
-	return 0, 0
 }
diff --git a/syncapi/sync/request.go b/syncapi/sync/request.go
index 620dfdcdb..c54f6db4e 100644
--- a/syncapi/sync/request.go
+++ b/syncapi/sync/request.go
@@ -21,6 +21,7 @@ import (
 	"math"
 	"net/http"
 	"strconv"
+	"sync"
 	"time"
 
 	"github.com/matrix-org/gomatrixserverlib"
@@ -101,6 +102,7 @@ func newSyncRequest(req *http.Request, device userapi.Device, syncDB storage.Dat
 		Rooms:             make(map[string]string),   // Populated by the PDU stream
 		WantFullState:     wantFullState,             //
 		MembershipChanges: make(map[string]struct{}), // Populated by the PDU stream
+		SyncMu:            &sync.Mutex{},
 	}, nil
 }
 
diff --git a/syncapi/sync/requestpool.go b/syncapi/sync/requestpool.go
index 766c58737..2affd1074 100644
--- a/syncapi/sync/requestpool.go
+++ b/syncapi/sync/requestpool.go
@@ -326,6 +326,8 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *userapi.
 				succeeded = err == nil
 				sqlutil.EndTransactionWithCheck(snapshot, &succeeded, &err)
 			}()
+			syncReq.SyncMu.Lock()
+			defer syncReq.SyncMu.Unlock()
 			return f(snapshot)
 		}
 
diff --git a/syncapi/types/provider.go b/syncapi/types/provider.go
index 9a533002b..614d99273 100644
--- a/syncapi/types/provider.go
+++ b/syncapi/types/provider.go
@@ -2,6 +2,7 @@ package types
 
 import (
 	"context"
+	"sync"
 	"time"
 
 	"github.com/matrix-org/gomatrixserverlib"
@@ -26,6 +27,7 @@ type SyncRequest struct {
 	MembershipChanges map[string]struct{}
 	// Updated by the PDU stream.
 	IgnoredUsers IgnoredUsers
+	SyncMu       *sync.Mutex
 }
 
 func (r *SyncRequest) IsRoomPresent(roomID string) bool {