diff --git a/ArchiSteamFarm/Collections/ConcurrentList.cs b/ArchiSteamFarm/Collections/ConcurrentList.cs
index d44f0b8f7..24fd3810f 100644
--- a/ArchiSteamFarm/Collections/ConcurrentList.cs
+++ b/ArchiSteamFarm/Collections/ConcurrentList.cs
@@ -49,12 +49,15 @@ public sealed class ConcurrentList<T> : IList<T>, IReadOnlyList<T> where T : not
 
 	public T this[int index] {
 		get {
+			ArgumentOutOfRangeException.ThrowIfNegative(index);
+
 			using (Lock.ReaderLock()) {
 				return BackingCollection[index];
 			}
 		}
 
 		set {
+			ArgumentOutOfRangeException.ThrowIfNegative(index);
 			ArgumentNullException.ThrowIfNull(value);
 
 			using (Lock.WriterLock()) {
diff --git a/ArchiSteamFarm/Collections/ObservableConcurrentDictionary.cs b/ArchiSteamFarm/Collections/ObservableConcurrentDictionary.cs
index 71f755323..8c19202e0 100644
--- a/ArchiSteamFarm/Collections/ObservableConcurrentDictionary.cs
+++ b/ArchiSteamFarm/Collections/ObservableConcurrentDictionary.cs
@@ -51,9 +51,14 @@ public sealed class ObservableConcurrentDictionary<TKey, TValue> : IDictionary<T
 	IEnumerable<TValue> IReadOnlyDictionary<TKey, TValue>.Values => Values;
 
 	public TValue this[TKey key] {
-		get => BackingDictionary[key];
+		get {
+			ArgumentNullException.ThrowIfNull(key);
+
+			return BackingDictionary[key];
+		}
 
 		set {
+			ArgumentNullException.ThrowIfNull(key);
 			ArgumentNullException.ThrowIfNull(value);
 
 			if (BackingDictionary.TryGetValue(key, out TValue? savedValue) && EqualityComparer<TValue>.Default.Equals(savedValue, value)) {