diff --git a/Implab/Diagnostics/TraceEvent.cs b/Implab/Diagnostics/TraceEvent.cs
--- a/Implab/Diagnostics/TraceEvent.cs
+++ b/Implab/Diagnostics/TraceEvent.cs
@@ -25,7 +25,7 @@ namespace Implab.Diagnostics {
         }
 
         public static TraceEvent Create(TraceEventType type, string format, params object[] args) {
-            return new TraceEvent(type, String.Format(format, args));
+            return new TraceEvent(type, String.Format(format ?? String.Empty, args));
         }
     }
 }