Identity Insert in EF6 Database-First

Question

I am attempting to convert data from an old database into a new database, where the primary key's value is important.

The code I currently have is as such (actual table name replaced with "TableName"):

using (var oldContext = new OldContext())
using (var newContext = new NewContext())
using (var transaction = newContext.Database.BeginTransaction())
{
    foreach (var item in GetConversionItems())
    {
        var newItem = Convert(item);
        newContext.TableName.Add(newItem);
    }

    try
    {
        newContext.Database.ExecuteSqlCommand("SET IDENTITY_INSERT [dbo].[TableName] ON;");
        newContext.SaveChanges();
        newContext.Database.ExecuteSqlCommand("SET IDENTITY_INSERT [dbo].[TableName] OFF");
        transaction.Commit();
    }
    catch (Exception e)
    {
        transaction.Rollback();
    }
}

and I do explicitly assign a value to the primary key.

public static TableName Convert(OldItem oldItem)
{
    ...
    newItem.ID = oldItem.CONTROL;
    ...
}

But I get the exception:

"Explicit value must be specified for identity column in table 'TableName' either when IDENTITY_INSERT is set to ON or when a replication user is inserting into a NOT FOR REPLICATION identity column."

I've tried making the actual database calls (IDENTITY_INSERT and SaveChanges()) inside the foreach loop, but the problem persisted.

The answers I was able to find for this issue were all for code-first.