Ошибка "Invalid Next control variable reference" - VB

Private Sub CommandButton1_Click()
Dim a, b, c, a1, b1, c1, S As Integer
a1 = 0
b1 = 0
c1 = 0
a = 10
b = 20
c = 70
S = 700
Do While (a1 * a + b1 * b + c1 * c = 700) And (a1 + b1 + c1 = 30)
For a1 = 0 To S / a
    For b1 = 0 To S / b
        For c1 = 0 To S / c
Next a1
    Next b1
        Next c1
Loop
TextBox1 = TextBox1 + Str(a1) + "/" + Str(b1) + "/" + Str(c1)
End Sub