diff --git a/src/OpcSession.cs b/src/OpcSession.cs
index d310ad0..1fec619 100644
--- a/src/OpcSession.cs
+++ b/src/OpcSession.cs
@@ -475,7 +475,7 @@ namespace Opc.Ua.Publisher
             try
             {
                 // find a subscription we could the node monitor on
-                OpcSubscription opcSubscription = OpcSubscriptions.DefaultIfEmpty(null).First(s => s.RequestedPublishingInterval == publishingInterval);
+                OpcSubscription opcSubscription = OpcSubscriptions.DefaultIfEmpty(null).FirstOrDefault(s => s.RequestedPublishingInterval == publishingInterval);
                 // if there was none found, create one
                 if (opcSubscription == null)
                 {
@@ -488,7 +488,7 @@ namespace Opc.Ua.Publisher
                 }
 
                 // if it is already there, we just ignore it, otherwise we add a new item to monitor.
-                OpcMonitoredItem opcMonitoredItem = opcSubscription.OpcMonitoredItems.DefaultIfEmpty(null).First(m => m.StartNodeId == nodeId);
+                OpcMonitoredItem opcMonitoredItem = opcSubscription.OpcMonitoredItems.DefaultIfEmpty(null).FirstOrDefault(m => m.StartNodeId == nodeId);
                 if (opcMonitoredItem == null)
                 {
                     // add a new item to monitor
@@ -608,7 +608,7 @@ namespace Opc.Ua.Publisher
             {
                 OpcSessionsSemaphore.Wait();
                 var opcSessions = OpcSessions.Where(s => s.Session != null);
-                var opcSession = opcSessions.Where(s => s.Session.ConfiguredEndpoint.EndpointUrl.Equals(session.ConfiguredEndpoint.EndpointUrl)).DefaultIfEmpty(null).First();
+                var opcSession = opcSessions.Where(s => s.Session.ConfiguredEndpoint.EndpointUrl.Equals(session.ConfiguredEndpoint.EndpointUrl)).DefaultIfEmpty(null).FirstOrDefault();
                 OpcSessionsSemaphore.Release();
                 if (!ServiceResult.IsGood(e.Status))
                 {
diff --git a/src/PublisherState.cs b/src/PublisherState.cs
index 573d0fb..ec59d17 100644
--- a/src/PublisherState.cs
+++ b/src/PublisherState.cs
@@ -155,7 +155,7 @@ namespace Publisher
                 try
                 {
                     OpcSessionsSemaphore.Wait();
-                    opcSession = OpcSessions.DefaultIfEmpty(null).First(s => s.EndpointUri == endpointUri);
+                    opcSession = OpcSessions.DefaultIfEmpty(null).FirstOrDefault(s => s.EndpointUri == endpointUri);
                     if (opcSession == null)
                     {
                         // do nothing if there is no session for this endpoint.