I am also stuck in the similar case... my surname on passport is left blank.. but i read in help section on CIC website that if your surname is left blank then write your full name under surname and write not applicable under given name... PLS COMMENT.!
"If you do not have a surname on your passport, travel or identity document, enter all your given name(s) in the surname field, write “not applicable” or “N/A” in the given name field."
Source: CIC help center question: """What do you mean by surname?"""
"""Your surname is your family name. It is sometimes called your “last name.” If your name is spelled differently on your official documents, please write your name as shown in your passport. When filling out applications, type your surname as it appears on your passport, travel or identity document or any letters you got from the visa office or case processing centre where you sent your application (even if the name is misspelled). Do not use initials.
If you do not have a surname on your passport, travel or identity document, enter all your given name(s) in the surname field, write “not applicable” or “N/A” in the given name field."""