Running Totals On Userforms And Negative Numbers

I have multiple textboxes that I've been trying to get a running total for. For example, I've got Amount1, amount2, amount3, etc. which should get totaled in a text box called AmountTotal. I would like this to automatically update once an amount is entered. The issue that I am having is that as soon as I try and put in a negative number in one of those boxes it returns a "type mismatch" error.

here's a sample of the code I am refering to.. How to write the code so it doesnt concatenate



Public Sub Amount2_change()
Dim a As Long
Dim b As Long

a = Amount1.Value
b = Amount2.Value

TextBox1 = a + b
End Sub..................