Using Vlookup to update a string

[synkarius]

Hi all. Here's my problem. I've declared some variables as follows:

Public activityone As String 
Public allactivities As Range

I'm using public variables because I'm lazy. I set allactivities:

Set allactivities = Worksheets("data").Range("G5:N14")

Problem: When I run the following code, I get a "Runtime error 1004: Application-defined or object-defined error":

activityone = Worksheets("data").Range("G19").Value
If MTdone = True Then
        activityone = Application.WorksheetFunction.VLookup(activityone, allactivities, 2, False)
End If

What am I doing wrong? It only happens when MTdone is True, so it has to be that line of code.