String Manipulation ?

I was given a script that was supposed to take a name field that was separated by commas and normalize it into last, first and middle name. My data looks like below in one fieldname called longname

crab,mike,Allen
Lota Weilly,Eric,M

My script to do this looks like

update ailoca
set last_name = substring (longname, 1, patindex( '%,%' , longname) -1 ),
first_name = substring (longname, patindex( '%,%' , longname) + 1, patindex( '% %', longname)-patindex( '%,%' , longname)),
middle_name = substring (longname, patindex( '% %', longname) + 1, len(longname)-patindex( '%,%' , longname))

My problem is that some people actually have 2 last names, not hyphenated, but 2. Whenever I have 2 names I get the following error

Server: Msg 536, Level 16, State 3, Line 1
Invalid length parameter passed to the substring function.
The statement has been terminated.

It seems to be related to the first name, I can comment out that update and it works

Thanks
Thanks