1 files changed, 5 insertions, 2 deletions
diff --git a/Emby.Server.Implementations/SyncPlay/SyncPlayManager.cs b/Emby.Server.Implementations/SyncPlay/SyncPlayManager.cs
index a2b9088708..b88ee33358 100644
--- a/Emby.Server.Implementations/SyncPlay/SyncPlayManager.cs
+++ b/Emby.Server.Implementations/SyncPlay/SyncPlayManager.cs
@@ -332,8 +332,11 @@ namespace Emby.Server.Implementations.SyncPlay
                 // Group lock required as Group is not thread-safe.
                 lock (group)
                 {
-                    // Make sure that session still belongs to this group.
-                    if (_sessionToGroupMap.TryGetValue(session.Id, out var checkGroup) && !checkGroup.GroupId.Equals(group.GroupId))
+                    // Make sure that session still belongs to this group. The lookup can fail
+                    // outright when the session left while this request was waiting on the group
+                    // lock, which is exactly the case this re-check exists to catch.
+                    if (!_sessionToGroupMap.TryGetValue(session.Id, out var checkGroup)
+                        || !checkGroup.GroupId.Equals(group.GroupId))
                     {
                         // Drop request.
                         return;