DotNetNuke - Restore Backup Issue

I'm currently having an issue restoring a backup of a DotNetNuke site. I have the .bak file. Using Microsoft SQL Server Management Studio 2014, I try to restore the .bak file to a database. However, a message remains at the top of the window: "No backupset selected to be restored." When I attempt to check the contents of the .bak, I receive the following error: "Object cannot be cast from DBNull to other types. (mscorlib)"

After doing some research, the most common cause of this error is attempting to restore a newer version of SQL to an older version of SQL. The problem is that I am using SQL Server 2014, so this certainly cannot be the issue. Please help!!!