diff --git a/ObservatoryFramework/EventArgs.cs b/ObservatoryFramework/EventArgs.cs
index de638a8..54a3e23 100644
--- a/ObservatoryFramework/EventArgs.cs
+++ b/ObservatoryFramework/EventArgs.cs
@@ -73,7 +73,7 @@ namespace Observatory.Framework
         /// <summary>
         /// A value which allows grouping of notifications together. For example, values &gt;= 0 &lt;= 1000 could be system body IDs, -1 is the system, anything else is arbitrary.
         /// </summary>
-        public int CoalescingId;
+        public int? CoalescingId;
     }
 
     /// <summary>