How do I get the sum of a field in a sub-form

How do I get the sum of a field in a sub-form

I am completely new to Zoho Creator.  Using the SUM documentation below, I attempted to use a formula field to sum a single numeric field in a sub-form.  I could never get it to work.  I then tried with all fields in a db with only one form.  Still no go, and I got the same errors.

Should sum() work in the Expression property of a Formula field?  If not, where do I use it to get a total for a field?  If the sum function is not the way to go, what is?

Here are results of various entries in Expression property
    Form - TestForm
    Numeric Field - NumberIn
    Formula Field - SumNumberIn

1 Numberin   Works, but of course it only shows each record's value, not a total
2 sum(NumberIn)   "Not able to find 'sum' function"
3 Testform.sum(NumberIn)  and SumNumberIn = Testform.sum(NumberIn)    "Expression not supported in formula"



Sum Function

//to sum the total value of a numeric field in the form 
<variable> = <Form_Name>.sum(<Numeric Field Name>);

//to sum specific records with a given criteria
<variable> = <Form_Name>[criteria expression].sum(<Numeric Field Name>);



where,
<variable> refers to a user defined variable name or a form field.

Example:

//to find total sales value 
Total_Sales = FormA.sum(Amount);

//to find the total sales value for Item="Printer"
ItemA = FormA[Item == "Printer"].sum(Amount);