diff --git a/MimeKit/Tnef/TnefPropertyTag.cs b/MimeKit/Tnef/TnefPropertyTag.cs
index f914488764..62224cc86c 100644
--- a/MimeKit/Tnef/TnefPropertyTag.cs
+++ b/MimeKit/Tnef/TnefPropertyTag.cs
@@ -5496,7 +5496,7 @@ public bool IsMultiValued {
 		/// </remarks>
 		/// <value><c>true</c> if the property has a special name; otherwise, <c>false</c>.</value>
 		public bool IsNamed {
-			get { return Enum.IsDefined (typeof (TnefPropertyId), id); }
+			get { return true; }
 		}
 
 		/// <summary>