Error 1004 when trying to modify and transfer data from one workbook to a new workbook

I am writing a macro for a report system which uses one file for each month, the macro transfers constants and cumulative statistics from the source workbook to a template file which has the same layout but contains no data. I am stuck when it comes to entering the start date for the new file. For example if the source workbook is for June with a start date of 1st June then the start date for the next workbook will be 1st July.

I have tried to use the following code:

OldDate = Workbooks(CFileName).Sheets("Setup").Range("Month_start").Value
NewDate = DateAdd("m", 1, OldDate)

Workbooks(NextDRS).Sheets("Setup").Range(Month_start).Value = NewDate

CfileName is the source workbook, NextDRS is the target. Ranges are named the same in both workbooks.

When the macro runs it get:
Error "1004"
Application-defined or object-defined error

As an amateur I do not understand what is causing this error I can transfer values from one worksheet to the other easily enough as in following example:

Workbooks(NextDRS).Sheets("Setup").Range("DetailsProj").Value _
= Workbooks(CFileName).Sheets("Setup").Range("DetailsProj").Value

I am obviously making a fundamental error but have been unable to solve it myself, any assistance would be greatly appreciated.

Cheers,
Jon

Dooooh....

Idiotically just noticed missing quotaion marks around the range name Month_start in the second instance!

Senior moment!!!