为什么在火星上得到“无效的程序计数器值”？

如何解决为什么在火星上得到“无效的程序计数器值”？

我正在尝试将十六进制转换为整数，但是我一直收到此错误。

错误是：错误：程序计数器值无效：0x00000000

我以此为指南https://github.com/exercism/mips/blob/master/exercises/hexadecimal/example.mips

.data
getHex: .asciiz "Enter 32-bit hex: "
userInput: .space 8
hexvals: .word 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,15


.text
li $v0,4 # put 4 in register $v0 in order to print whats in the address of $a0
la $a0,getHex # put the address of label getHex in $a0 in order for string with null to be printed
syscall

li $v0,8
la $a0,userInput
li $a1,9
syscall


hex_convert:
        li      $v0,0                  # Reset accumulator to 0.
        la      $t0,hexvals           # Load address of lookup into register.
loop:
        lb      $t1,0($a0)             # Load a character,beq     $t1,$zero,end         # if it is null then return.
        sll     $v0,$v0,4             # Otherwise first shift accumulator by 4 to multiply by 16.
        addi    $t2,$t1,-48           # Then find the offset of the char from '0'
        sll     $t2,$t2,2             # in bytes,addu    $t2,$t0           # use it to calcute address in lookup,lw      $t3,0($t2)             # retrieve its integer value,addu    $v0,$t3           # and add that to the accumulator.
        addi    $a0,$a0,1             # Finally,increment the pointer
        j       loop                    # and loop.

end:
        jr $ra