I got a error: Runtime exception at 0x004000c4:invalid integer input(syscall 5). What should I change?

.data       
############################################    
.eqv    ISO_LF      10  # Line feed (newline)
.eqv   SYS_PRINT_CHAR   11
.align 2
userinput: .space 100
head:   .word 0
tail:   .word 0
ask: .asciiz "Enter a number: "
message: .asciiz"\nyour enter number is: "
    #########################################################################
    # Receiver control.  1 in bit 0 means new char has arrived.  This bit
    # is read-only, and resets to 0 when CONS_RECEIVER_DATA is read.
    # 1 in bit 1 enables hardware interrupt at interrupt level 1.
    # Interrupts must also be enabled in the coprocessor 0 status register.

.eqv   CONS_RECEIVER_CONTROL    0xffff0000
.eqv   CONS_RECEIVER_READY_MASK 0x00000001
.eqv   CONS_RECEIVER_DATA    0xffff0004

.eqv   DISPLAY_CONTROL 0xffff0008
.eqv   CONS_DISPLAY_MASK 0x00000001
.eqv   DISPLAY_DATA   0Xffff000c 
    # Main body
.text

main:   
    lui $t0,0xffff  
    li $v0,4
    la $a0,ask
    syscall 
    li $v0,5
    syscall
    li $v0,4
    la $a0,message
    syscall    
    li $v0,1
    syscall
    jal MakeList
    li $v0,10
    syscall
        # Spin-wait for key to be pressed\
waitinloop:
    addi $t0,$zero,20
    addi $t1,$zero,40

    slt $s0,$t0,$t1
    bne $s0,$zero,ask

    li $v0,10
    syscall

waitoutloop:
    addi $t0,$zero,10
    addi $t1,$zero,30

    slt $s0,$t0,$t1
    bne $s0,$zero,message

    li $v0,10
    syscall

key_wait:
        lw      $t0, CONS_RECEIVER_CONTROL
        andi    $t0, $t0, CONS_RECEIVER_READY_MASK  # Isolate ready bit
        beqz    $t0, key_wait

        # Read in new character from keyboard to low byte of $a0
        # and clear other 3 bytes of $a0


        lbu     $a0, CONS_RECEIVER_DATA
display_wait:
        lw $t0, DISPLAY_CONTROL
        andi $t0, $t0, CONS_DISPLAY_MASK
        beqz $t0,display_wait

        sw  $a0, DISPLAY_DATA  #store it to the display data buffer

        # Print character and newline  to standard out
            # to show it works
        li      $v0, SYS_PRINT_CHAR
        syscall

        li      $a0, ISO_LF
        li      $v0, SYS_PRINT_CHAR
        syscall
        j key_wait

MakeList:

    move $t7, $0        #initialize to 0, this will be the head
    move $t8, $0         #tail pointer  

    li $v0, 5 #read an int and put it inot $t1
    syscall
    move $t1,$v0    

MakeList_1: 
    beqz $t1, MakeList_5    
        li $a0,8      #malloc 8 bytes from the heap
        li $v0,9
        syscall 

        sw $t1, 4($v0)  #put int in node
        sw $0, ($v0)    #initialize next to null

        bnez $t7 MakeList_2   
            #empty list
                move $t7, $v0  #head = malloc(2)
                move $t8, $t7  #tail = head
                b MakeList_3
MakeList_2: #else   not empty, add to end
            sw $v0, ($t8)   #link to tail.next, first word in node
            lw $t8, ($t8)   #tail = tail.next

MakeList_3:         
        li $v0, 5
        syscall
        move  $t1,$v0   #read an integer
    b MakeList_1                                

MakeList_5:
    sw $t7,head
    sw $t8,tail                                                                                                 
    jr $ra

factorial: 
    add $sp,$sp,8
    sw $ra, ($sp)
    sw $s0,4($sp)

    li $v0,1
    beqz,$a0, done

    move $s0, $a0
    add $a0,$a0,-1
    jal factorial
    mul $v0,$v0,$s0

done:
    lw $ra,($sp)
    lw $s0 ,4($sp)
    add $sp,$sp,8
    jr $ra