get value from parent form using javascript

I am working on CRM 2011 online.
I would like to get field(allotmenttypecode) from parent entity (contract) into child entity form(contract line).
When the form of contract line is open (onload) I am using the script below to get value from parent.
          window.parent.opener.Xrm.Page.data.entity.attributes.get("allotmenttypecode").getValue();

but it always return error message: window.parent.opener.Xrm.Page.data.entity is null or not an object.
Please help me out.