diff --git a/Xbim.IO.MemoryModel/MemoryModel.cs b/Xbim.IO.MemoryModel/MemoryModel.cs
index 91451193c..315a90132 100644
--- a/Xbim.IO.MemoryModel/MemoryModel.cs
+++ b/Xbim.IO.MemoryModel/MemoryModel.cs
@@ -316,6 +316,7 @@ public static MemoryModel OpenReadStep21(string file, ILogger logger = null, Rep
         /// <param name="logger">Logger</param>
         /// <param name="progressDel">Progress delegate</param>
         /// <param name="ignoreTypes">A list of ifc types to skip</param>
+        /// <param name="allowMissingReferences">Allow referenced entities that are not in the model, default false</param>
         /// <returns>New memory model</returns>
         public static MemoryModel OpenReadStep21(Stream stream, ILogger logger = null, ReportProgressDelegate progressDel = null,
            IEnumerable<string> ignoreTypes = null, bool allowMissingReferences = false)