diff --git a/src/Orleans.Core/Utils/ObserverManager.cs b/src/Orleans.Core/Utils/ObserverManager.cs
index 238c38e393..71d023e6fd 100644
--- a/src/Orleans.Core/Utils/ObserverManager.cs
+++ b/src/Orleans.Core/Utils/ObserverManager.cs
@@ -153,7 +153,7 @@ public async Task Notify(Func<TObserver, Task> notification, Func<TObserver, boo
         {
             var now = GetDateTime();
             var defunct = default(List<TIdentity>);
-            foreach (var observer in _observers)
+            foreach (var observer in _observers.ToList())
             {
                 if (observer.Value.LastSeen + ExpirationDuration < now)
                 {
@@ -208,7 +208,7 @@ public void Notify(Action<TObserver> notification, Func<TObserver, bool> predica
         {
             var now = GetDateTime();
             var defunct = default(List<TIdentity>);
-            foreach (var observer in _observers)
+            foreach (var observer in _observers.ToList())
             {
                 if (observer.Value.LastSeen + ExpirationDuration < now)
                 {
@@ -257,7 +257,7 @@ public void ClearExpired()
         {
             var now = GetDateTime();
             var defunct = default(List<TIdentity>);
-            foreach (var observer in _observers)
+            foreach (var observer in _observers.ToList())
             {
                 if (observer.Value.LastSeen + ExpirationDuration < now)
                 {