Error in compiling a working MIPS shell code in MARS (MIPS simulator) about Syscall

Question

I want to compile an assembly code of a shell code of MIPS in the MARS simulator shell code example is:

 lui     $6,0x4321
 ori     $6,$6,0xfedc
 lui     $5,0x2812
 ori     $5,$5,0x1969
 lui     $4,0xfee1
 ori     $4,$4,0xdead
 li      $2,4088
 syscall 0x40404

and at compile time it gives this error: "syscall": Too many or incorrectly formatted operands. Expected: syscall

and at real compile when I placed its code in another true program code, it gives again error: shell_reboot.s:124: Error: illegal operands `syscall'

What I have to do? what is the problem and how it should be?

shell code source: http://packetstormsecurity.com/files/107735/Linux-MIPS-reboot-Shellcode.html