diff --git a/test/Docfx.Tests.Common/TestListenerScope.cs b/test/Docfx.Tests.Common/TestListenerScope.cs
index 9c9764168e8..9607a8c084c 100644
--- a/test/Docfx.Tests.Common/TestListenerScope.cs
+++ b/test/Docfx.Tests.Common/TestListenerScope.cs
@@ -24,7 +24,7 @@ public void Flush() { }
     public void WriteLine(ILogItem item)
     {
         if (item.LogLevel >= _logLevel)
-            s_items.Value.Add(item);
+            s_items.Value?.Add(item);
     }
 
     public IEnumerable<ILogItem> GetItemsByLogLevel(LogLevel logLevel)