Email based on checkbox - If function

I'm trying to send an automated mail based on whether a checkbox is checked.

The code works perfectly without the If function. But with it, I get:

Error 438: Object doesn't support this property or method.

I'd rather keep the If function so the mail only gets sent by checking the box. Without the If function, the mail gets sent when unchecking as well.

Sub Checkbox1_Click()
    Dim OutLookApp As Object
    Dim Mail As Object
    Dim subject_ As String
    Dim body_ As String
    subject_ = "Something"
    body_ = "Something else"
    If Sheets("Sheet1").CheckBox1.Value = True Then
        Set OutLookApp = CreateObject("Outlook.Application")
        Set Mail = OutLookApp.CreateItem(0)
        Application.DisplayAlerts = False
        With Mail
            .Subject = subject_
            .Body = body_
            .To = "email"
            .CC = "otheremail"
            .Importance = 2
            .Send
        End With
        Application.DisplayAlerts = True
    End If
End Sub