問一個vb6簡單的遞迴程式

1+2+3+...N
以下是程式
Sub Cd1ck()
Dim i As Integer
i = InputBox("請輸入所要計算的連加數值")
Debug.Print "1+2+....+"; i - 1; "+"; i; "="; sum(i)
End Sub

Function sum(ByRef N As Integer) As Integer
If N = 1 Then sum = 1 Else sum = N + sum(N - 1)
End Function

請問此句
If N = 1 Then sum = 1 Else sum = N + sum(N - 1)
sum=1為何不能改成sum(N)=1呢？
謝謝