求助出错原因，可以运行但就是在运行过程中会出现偶然错误

Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
Dim a%, b%, c%, m%, i%, s$, n$
Randomize()
s = ""
i = 0
Do
a = Int(4 * Rnd() + 1)
b = Int(4 * Rnd() + 1)
c = Int(4 * Rnd() + 1)
If a <> b And b <> c And a <> c Then
m = a * 100 + b * 10 + c
n = Str(m)
If InStr(s, n) = 0 Then
s = s & n & ""
i = i + 1
End If
Loop Until i >= 24
Label1.Text = s
End Sub
就是运行过程中的即时窗口会出现在 System.InvalidCastException 中第一次偶然出现的“Microsoft.VisualBasic.dll”类型的异常