Adding custom document properties to word 2007

I am currently on .Net 4.5.1 . To create Custom document properties on word i used the dynamic keyword. as

dynamic cstmDocProp = mScribeShell.ScribeShell.ActiveWordDoc.CustomDocumentProperties;

string s = Convert.ToString(cstmDocProp[mConstants.g_sPROPERTY_DOCUMENT_INDEX].Value); 

On doing this i am getting an exception as: Value doesnt fall within the expected range of system.dynamic.

I also tried using :

Microsoft.Office.Core.DocumentProperties cstmDocProp = (Microsoft.Office.Core.DocumentProperties)mScribeShell.ScribeShell.ActiveWordDoc.CustomDocumentProperties;

How do i resolve this ? or is there a different way i can access/create document properties both built-in & custom for word 2007 ?

Thanks/Saurav.

Suarav