有问题要问情高手回答一下

Private Sub Form_Click()
Dim i, b As Doublea = Val(InputBox("", "", 0))
For i = 2 To a - 1
If a Mod i = 0 Then
Exit For
Next i
If i > a - 1 Then
Print "yes"
Else
Print "no"
End If
End Sub
为什么会出现next 没有 for呢？？