Найдите пожалуйста, ошибку в коде калькулятора

[Виктория67850]

Dim a As Double
Dim b As String
Dim mem As Double
Private Sub CommandButton1_Click()
TextBox1.Text = TextBox1.Text & "7"

End Sub


Private Sub CommandButton10_Click()
TextBox1.Text = TextBox1.Text & "0"

End Sub


Private Sub CommandButton12_Click()
a = Val(TextBox1.Text)
TextBox1.Text = " "
b = "+"
End Sub

Private Sub CommandButton13_Click()
If b = "" Then
a = Val(TextBox1.Text)
ElseIf b = " + " Then
a = a + Val(TextBox1.Text)
ElseIf b = " - " Then
a = a - Val(TextBox1.Text)
ElseIf b = " * " Then
a = a * Val(TextBox1.Text)
ElseIf b = " / " Then
a = a / Val(TextBox1.Text)
End If
TextBox1.Text = " "
b = " - "


End Sub

Private Sub CommandButton14_Click()
If b = " + " Then
TextBox1.Text = TextBox1.Text + Val(a)
ElseIf b = " - " Then
TextBox1.Text = Val(a) - TextBox1.Text
ElseIf b = " * " Then
TextBox1.Text = Val(a) * TextBox1.Text
ElseIf b = " / " Then
TextBox1.Text = Val(a) / TextBox1.Text
End If
a = " "
b = " "


End Sub

Private Sub CommandButton15_Click()
TextBox1.Text = " "
End Sub

Private Sub CommandButton16_Click()
mem = mem + Val(TextBox1.Text)
End Sub

Private Sub CommandButton17_Click()
mem = mem - Val(TextBox1.Text)
End Sub

Private Sub CommandButton2_Click()
TextBox1.Text = TextBox1.Text & "8"

End Sub

Private Sub CommandButton3_Click()
TextBox1.Text = TextBox1.Text & "9"

End Sub

Private Sub CommandButton4_Click()
TextBox1.Text = TextBox1.Text & "4"
End Sub

Private Sub CommandButton5_Click()
TextBox1.Text = TextBox1.Text & "5"
End Sub

Private Sub CommandButton6_Click()
TextBox1.Text = TextBox1.Text & "6"

End Sub

Private Sub CommandButton7_Click()
TextBox1.Text = TextBox1.Text & "1"

End Sub

Private Sub CommandButton8_Click()
TextBox1.Text = TextBox1.Text & "2"

End Sub

Private Sub CommandButton9_Click()
TextBox1.Text = TextBox1.Text & "3"

End Sub