[RESOLVED] Help textbox input error

[langamer101]

when I compile and run my code then input or type a letter in txtCS_Change() I get an (run-time error "13": type mismatch)
I want to know how to create a msgbox if I input or type a letter in the texbtox

Dim X As Double
Dim b As Double
Dim c As Double
Dim d As Double
Dim e As Double
Dim a As Double

Private Sub Command2_Click()

End Sub

Private Sub cmdCOMPUTE_Click()
a = Val(txtCS.Text)
b = Val(txtHLAB.Text)
c = Val(txtLEC.Text)
d = Val(txtME.Text)
e = Val(txtQ.Text)
X = (Val(a) + Val(b) + Val(c) + Val(d) + Val(e)) / 5
txtPG = X
MsgBox " the periodic grade is " & X & "", vbInformation, "RESULT"

End Sub

Private Sub cmdRESET_Click()
txtCS = ""
txtHLAB = ""
txtLEC = ""
txtME = ""
txtPG = ""
txtQ = ""
txtCS.SetFocus
End Sub

Private Sub txtCS_Change()
If txtCS > 100 Then
MsgBox "Grade is greater than 100, Pls re-enter", vbInformation, "ERROR"
End If
End Sub

Private Sub txtHLAB_Change()
If txtHLAB > 100 Then
MsgBox "Grade is greater than 100, Pls re-enter", vbInformation, "ERROR"
End If
End Sub

Private Sub txtLEC_Change()
If txtLEC > 100 Then
MsgBox "Grade is greater than 100, Pls re-enter", vbInformation, "ERROR"
End If
End Sub

Private Sub txtME_Change()
If txtME > 100 Then
MsgBox "Grade is greater than 100, Pls re-enter", vbInformation, "ERROR"
End If
End Sub

Private Sub txtQ_Change()
If txtQ > 100 Then
MsgBox "Grade is greater than 100, Pls re-enter", vbInformation, "ERROR"
End If
End Sub