Error after converting standard workbook to add in

Hi
I have recently converted a standard workbook (.xlsm) to an addIn(.xlam) file and it now fails when attempting call and activate one of the associated worksheets with the errors below. It appears as if the worksheets are now out of scope.

Run-time error '1004':
Method 'Sheets' of object '_Global' failed

The code I am using is
------------------------------------------------------------
Private Sub Command_Next1_Click()
Worksheets("Sheet1").Activate
End Sub-------------------------------------------------------------

I have tried changing the sheet reference format to those below but still get the same error.
Sheets(1).Activate
Worksheets("lookups").Activate
Any advice on this issue would be very much appreciated.
Regards
Iain

Hi Norie

Thanks for the quick response.

The original wokbook is called ExpensesWB.xlsm and converted ExpensesWB.xlam both use the default “ThisWorkbook” workbook. When I prefix with this I get the error.
“Method or data member not found”