HPSM Connect-It - Email to Ticket intergration

Hi Experts,

I am currently using a Connect-It scenario to EmailFetch and send them to ServiceManagerWebService to CreateInteraction. The issue is it is now making tickets for every single email, so I am creating a Mapping Script to filter Replies by the RE: tag in the subject line so to ignore those. (I plan to also make another scenario to update current tickets based on the RE: tags and the rest of the subject)

The issue I am having is that my script is giving me an error that the Types are mismatched: 

2016/10/06 10:43:56.000 0 1 (0) Runtime error - Type mismatch.

I was hoping someone here might be able to help me out, as I have little knowledge of VB/Connect-It Basic. I have read through a lot of the documentation for Connect-It 9.60. I believe the issue is with the Substring portion, as I don't know if Connect-It basic understands that.

Dim Left as String
Left = LeftPart([Subject]," ",1)
If Substring([Subject] - 1,3)=Left Then
PifIgnoreDocumentMapping
Else
Retval=[Subject]
End If