How to create a subtotal on a subform and display it on main form in Access 2010

I am learning Access 2010, and am trying to build a database as part of my learning experience. The database I am building is one to track my family's budget/finances. I have a table called Transactions (parent) and another one called TransactionDetails (child). I have built a form (called FORM_ExpenseTransactions) with a subform (FORM_ExpenseTransactionDetails) to enter data into these fields. The subform can handle multiple expense categories for any transaction.

What I'm trying to do is display a subtotal of the expense amount (subform field called "ExpenseAmount"). In the subform footer, I have created a textbox and entered, "=Sum([ExpenseAmount])" in the Control Source and named it, "TotalExpense".

Then I created another textbox in the main form and entered, "=[FORM_ExpenseTransactionDetails].[Form]![TotalExpense]" in the Control Source.

The textbox on the main form just says, "#Name?". What did I do wrong?

I have attached a database with a handful of ficticious transactions if that would help.

Thanks in advance for any words of wisdom from experienced Access gurus!
Doug

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