diff --git a/src/process.tests/DiagnosticListenerTests.cs b/src/process.tests/DiagnosticListenerTests.cs
index 7549a66..fd3ec80 100644
--- a/src/process.tests/DiagnosticListenerTests.cs
+++ b/src/process.tests/DiagnosticListenerTests.cs
@@ -122,7 +122,8 @@ internal sealed class DiagnosticListenerTests
 			_name = name;
 		}
 
-		public IReadOnlyList<KeyValuePair<string, Hashtable>> Events => _events;
+		public IReadOnlyList<KeyValuePair<string, Hashtable>> Events => _events.AsReadOnly();
+
 		public Action<string, Hashtable>? ContextChanged { get; set; }
 
 		public void OnNext(DiagnosticListener value)