If you use the Merge with Attachments (attachments are not essential) utility on my MergeTools add-in, you should avoid the problem.
You can download the MergeTools – 20140218.dotm Add-in that I created from the following page of my One Drive:
http://bit.ly/1hduSCB
The file needs to be saved in the Word Startup folder. In Windows XP the default location for that folder is
C:\Documents and Settings\[User Name]\Application Data\Microsoft\Word\STARTUP
In Windows Vista and Windows 7, 8 or 8.1 it is
C:\Users\[User Name]\AppData\Roaming\Microsoft\Word\STARTUP
If you do not see the AppData folder: -
In Windows 7, - In Windows Explorer, click on the Organize drop down and then on Folder and search options and in the Folder Options dialog, go to the View tab and select
the item "Show hidden files, folders, and drives". While there, it is a good idea to uncheck the box of "Hide extensions for known file types".
In Windows 8 and 8.1, in the File Explorer, click on Options on the View tab of the ribbon and then on the View tab in the dialog that appears and select the item "Show hidden files, folders, and drives". While there, it is a good idea to uncheck the box of
"Hide extensions for known file types".
When that has been done and Word is started\re-started, the tab shown below will be added to the Ribbon:
One thing to note is that the field names in the data source must contain only alphanumeric characters (No @,#,$,%,&,(,), etc) and the field names must not start with a numeric character
(0-9).
You may also want to download the Merging with Attachments document that is also on that page which explains how the system is used.
It is not actually necessary to have separate attachments as the facility can be used to send just the documents created by the merge itself as attachments, either as the body of the message itself or in the form of Word files or .pdf files.