Chapter 7 Firewall for FortiOS 5.0 : GUI & CLI - What You May Not Know : Character Restrictions
  
Character Restrictions
A name can contain numbers (0-9), uppercase and lowercase letters (A-Z, a-z), spaces, and the special characters - and _. Other characters are not allowed
The special characters < > ( ) # " ' are allowed only in the following fields:
Passwords
Replacement message
Firewall policy description
IPS customized signature
Antivirus blocked file pattern
Web Filter banned word
Spam filter banned word
interface PPPoE client user name
modem dialup account user name
modem dialup telephone number
 
Where you would normally be tempted to use spaces in a name try using the "-" or "_". There are a few name fields were it is not an issue but most of them will trigger serious and unpredictable issues if there is a space in the name field of the object.