Using SQL Azure for session state throws a cast error

I have a web application running on an Azure VM using SQL Azure for persistence. I just added a second SQL Azure database for ASPState. I believe that I have set up the ASPState database correctly but certain transaction cause aUnable to cast object of type 'System.DBNull' to type 'System.Byte[]'.

Here is the TempSessions table schema

TABLE[dbo].[ASPStateTempSessions](

NVARCHAR(88)   NOTNULL,

DATETIME        DEFAULT(getutcdate())NOTNULL,

DATETIME        NOTNULL,

DATETIME        NOTNULL,

DATETIME        NOTNULL,

INT             NOTNULL,

INT             NOTNULL,

BIT             NOTNULL,

VARBINARY(7000)NULL,

IMAGE           NULL,

INT             DEFAULT((0))NOTNULL,

PRIMARYKEYCLUSTERED([SessionId]ASC)

NONCLUSTEREDINDEX[Index_Expires]

ON[dbo].[ASPStateTempSessions]([Expires]ASC);