Use Isolation level in LINQ

I try to use isolation level with L2E, but I've this problem:

The transaction specified for TransactionScope has a different IsolationLevel than the value requested for the scope. Parameter name: transactionOptions.IsolationLevel

And I dont understand why.

This error is displayed when I call my dedicated method:

public static int insertDB(InsertDbDelegate methodThatWriteDB, IsolationLevel isoLevel, MasterPage senderPage)
    {
        int objectsAdded = 0;
 
        try
        {
            using (var scope = new TransactionScope(TransactionScopeOption.Required, new TransactionOptions() { IsolationLevel = isoLevel }))
            {
                using (geststockEntities gse = new geststockEntities(cs_admin))
                {
                    methodThatWriteDB(gse);
                    objectsAdded = gse.SaveChanges();
                    if (objectsAdded == 0) throw new Exception();
                    scope.Complete();
                    gse.Connection.Close();
                    addUserMessage(senderPage, "Insertion OK");
                }
            }
        }
        catch (Exception ex)
        {
            addErrorMessage(senderPage, "Error when connecting to the database." + ex.Message);
        }
 
        return objectsAdded;
    }

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