СЛАУ метод Гаусса Проверка некорректности данных - VB

Sub Gauss()
Dim a(1 To 3, 1 To 3) As Double
Dim b(1 To 3) As Double
Dim x(1 To 3) As Double
Dim z As Double
Dim r As Double
Dim g As Double
Dim i As Integer
Dim j As Integer
Dim k As Integer
a(1, 1) = Val(TextBox1.Text)
a(1, 2) = Val(TextBox2.Text)
a(1, 3) = Val(TextBox3.Text)
a(2, 1) = Val(TextBox5.Text)
a(2, 2) = Val(TextBox6.Text)
a(2, 3) = Val(TextBox7.Text)
a(3, 1) = Val(TextBox9.Text)
a(3, 2) = Val(TextBox10.Text)
a(3, 3) = Val(TextBox11.Text)
b(1) = Val(TextBox4.Text)
b(2) = Val(TextBox8.Text)
b(3) = Val(TextBox12.Text)
'Приведение матрицы к треугольному виду
For k = 1 To 3
  For j = k + 1 To 3
    r = a(j, k) / a(k, k)
      For i = k To 3
        a(j, i) = a(j, i) - r * a(k, i)
      Next i
      b(j) = b(j) - r * b(k)
  Next j
Next k
'Вычисление корней
For k = 3 To 1 Step -1
  r = 0
  For j = k + 1 To 3
    g = a(k, j) * x(j)
    r = r + g
  Next j
  x(k) = (b(k) - r) / a(k, k)
Next k
'Вывод корней
TextBox13.Text = x(1)
TextBox14.Text = x(2)
TextBox15.Text = x(3)
End Sub