Run time error 91 when opening workbook via vba

Hi - I'm trying to pull data from various workbooks using vba, but seem to get a run time error 91 after the first workbook has opened. This is my code:

Sub ImportBnHData()

Dim strFilePath As String
Dim wbTemp As Workbook
Dim i As Integer


For i = 1 To 13
strFilePath = [BacktestingDataFolder] & [BnHDataFiles].Offset(i, 1) & ".xlsx"
wbTemp = Workbooks.Open(strFilePath)
wbTemp.Sheets("Summary").Activate

Next I

[BacktestingDataFolder] & [BnHDataFiles] are named ranges in the spreadsheet that I am using. Any help would be great!

Thanks,
P