Validating userform combobox selection with vba and getting an error

So I am using the following code to let the user know they need to select or type an option in the dropdown. It works just fine when I define the name of the combobox but when I try to set it up as a sub I get a "Run-time error '424':" "Object Required". What am I missing?

Sub List_Select_Verify(CB)
    If CB = "" Then
        Exit Sub
    ElseIf CB.ListIndex = -1 Then
        CB.Value = ""
        MsgBox "You must select a valid option from the dropdown list.", vbCritical
    End If
End Sub