Code provided by andreasbotsikas from GIT repository:

- If custom property value is null insert empty string (This will avoid crashing the AddProperty. Another way would be to throw an application exception, but it's much more intuitive to set an empty string instead.)
- Check if stream can seek (This will allow devs to save the document directly on a System.Web.HttpResponseStream without the need to create yet another memory stream.)
- Fix duplicate custom properties (Check if property exists ignoring case as it would produce a corrupted docx otherwise.)

DocX/DocX.cs

@@ -3275,13 +3275,17 @@ namespace Novacode
             }
             else
             {
-                // Set the length of this stream to 0
-                stream.SetLength(0);
+                if (stream.CanSeek) // 2013-05-25: Check if stream can be seeked to support System.Web.HttpResponseStream
+                {
+                    // Set the length of this stream to 0
+                    stream.SetLength(0);
 
-                // Write to the beginning of the stream
-                stream.Position = 0;
+                    // Write to the beginning of the stream
+                    stream.Position = 0;
+                }
 
                 memoryStream.WriteTo(stream);
+                memoryStream.Flush();
             }
             #endregion
         }
@@ -3608,10 +3612,11 @@ namespace Novacode
                 pid = pids.Max();
 
             // Check if a custom property already exists with this name
+            // 2013-05-25: IgnoreCase while searching for custom property as it would produce a currupted docx.
             var customProperty =
             (
                 from d in customPropDoc.Descendants()
-                where (d.Name.LocalName == "property") && (d.Attribute(XName.Get("name")).Value == cp.Name)
+                where (d.Name.LocalName == "property") && (d.Attribute(XName.Get("name")).Value.Equals(cp.Name, StringComparison.InvariantCultureIgnoreCase))
                 select d
             ).SingleOrDefault();
 
@@ -3628,7 +3633,7 @@ namespace Novacode
                     new XAttribute("fmtid", "{D5CDD505-2E9C-101B-9397-08002B2CF9AE}"),
                     new XAttribute("pid", pid + 1),
                     new XAttribute("name", cp.Name),
-                        new XElement(customVTypesSchema + cp.Type, cp.Value)
+                        new XElement(customVTypesSchema + cp.Type, cp.Value ?? "")
                 )
             );
 
@@ -3637,7 +3642,7 @@ namespace Novacode
                 customPropDoc.Save(tw, SaveOptions.None);
 
             // Refresh all fields in this document which display this custom property.
-            UpdateCustomPropertyValue(this, cp.Name, cp.Value.ToString());
+            UpdateCustomPropertyValue(this, cp.Name, (cp.Value ?? "").ToString()); 
         }
 
         /// <summary>