Excel VBA Run-time error 13 - mismatch when box is blank

Question

I am stuck in VBA. I have tried other solutions on the website but still can't get it right. I am using multiple modules and forms to get some information input into cells of Excel. But, when the msgBox is left blank it gives me a type 13 mismatch error. I have tried isNull but don't fully understand how to use it.

Any help would be much appreciated, and please keep any answers as simple as possible as I am a novice programmer at best. Thanks

Sub GetEndCostGateFees()
    Dim qtyGateFees As Double
    Dim msg As String

Const MinCost As Integer = 0
Const MaxCost As Integer = 200

msg = "Please enter the cost, per tonne, of Gate fees "

Do
    qtyGateFees = InputBox(msg, "Gate Fees")

    If IsNull(qtyGateFees) Then
        MsgBox ("Please enter a value. Enter 0 if none")
        End If

    If IsNumeric(qtyGateFees) Then
        If qtyGateFess >= MinCost And qtyGateFees <= MaxCost Then Exit Do
        End If
        msg = "Please enter a valid number"
        msg = msg & vbNewLine
        msg = msg & "Please enter number between " & MinCost & " and " & MaxCost
        Loop

Sheet25.Range("B43").Value = qtyGateFees

End Sub

Answer 1

If you want the user to enter only Numeric Input then use Application.InputBox with Type:=1

Sub sample()
    Dim Ret As Variant
    Dim msg

    msg = "Please enter the cost, per tonne, of Gate fees "

    Ret = Application.InputBox(msg, "Gatefees", Type:=1)

    If Ret <> False Then
        '
        '~~> Rest of your code here
        '
    End If
End Sub