The first line should be:
If IsNull(FieldName) Or FieldName = 0 Then Exit Sub

Replace FieldName with the name of your field.

Don't use Sum as a field name as it is a reserved word. It is the name of a function.

The error message still pops up.

When your form has no record, all your fields have no value.  When you reference one of your fields with Me.Field1, you will also get the "expression as no value" error.  When you try to sum a field with "no value," your sum field will, of course, also has no value, and that's why you got this error.  "No value" doesn't mean null, so IsNull() doesn't catch it. The workaround for this is to first check if your form has any record, such as with the DCount() function, or with the Form.Recordset.Recordcount property.

I don't think that would be the problem since the form gets its value from a query.

Is this a calculated field?

Please post some screenshots of both your Form and your Query.

Yes.

The second query is based on the first one. Everything works fine but the error message when the table has no value in it.

Does this work for you?

    Dim myCount As Long
    
    myCount = DCount("*", "OrderSumQ")
    If myCount = 0 Then Exit Sub
    
    If IsNull(OrderSum) Or OrderSum = 0 Then Exit Sub
    
    If OrderSum > 400 Then
        If OrderSum.BackColor = vbWhite Then
            OrderSum.BackColor = vbRed
            OrderSum.ForeColor = vbWhite
        Else
            OrderSum.BackColor = vbWhite
            OrderSum.ForeColor = vbBlack
        End If
    End If

Hi Masih, whether it comes from a query or a form, it will still give the "expression has no value" error.  If there is no record to be summed, then the sum is not zero, not null, but "no value".  In Kevin's code above, the conditions IsNull(OrderSum) or OrderSum=0 will always be false because OrderSum will never be null or zero when there is no record to sum.  You can easily prove this with the immediate window.  Set a break on point that line.  When the execution pauses there, type:

    ?OrderSum

in the immediate window.  You will get the "expression has no value" error just like that.

Thanks. Great. It seems that Kevins code worked just fine.