If Else Statement Error

I have an input box where a user inserts a contract reference. This then is used by Excel to do various actions.
What I want to be able to do is include an 'else' so that if the user inserts an invalid reference the resulting '#N/A' error doesn't give me a 'Runtime Error 13'.
I am guessing that I need to put an else Iserror code in somehow but since this is the first bit of VB I have ever written without using the record macro button in Excel I'm a bit stumped.
Code so far is as follows:

Sub Selection()

crit = Inputbox("Please Enter a Contract Reference Number:", "Enter Contract Reference")
Range("B5").Select
Range("B5") = crit

If Range("C13") = "S/O" Then

MsgBox prompt:="Warning! This is a Scheduled Order. Please ensure you have input the Contract Reference correctly.", Buttons:=vbOKOnly + vbCritical, Title:="WARNING!"

' I am guessing my else iserror part should be in here...

End If

End Sub

Thanks in advance.

You could do something like

On Error Resume Next