I'm writing MIPS for the pic below, but there's an error: Error in : invalid program counter value: 0x00000000

and this is my codes:

.data
mesg1: .asciiz "The value you entered is less than zero. This program only works with values greater than or equal to zero."
mesg2: .asciiz "You input: "
nl: .asciiz "\n"
mesg3: .asciiz "The factorial is: "
mesg4: .asciiz "do you like to do this again? "
userInput: .space 10
yes: .byte 'Y'

.text
main:
li $v0, 5               #   user input
syscall
move $t0, $v0   
li $t2, 0               #   load 0 into t2 for comparsion
li $t3, 1               #   load 1 into t3 for bne
slt $t1, $t0, $t2           #   store result in t1
beq $t2, $t3, error         #   if input is smaller than 0, jump to error
bne $t2, $t3, Fact          #   if input is greater or equal to 0, call factorial function

li $v0, 4               #   print string
la $a0, mesg2               #       print out mesg2
syscall  

li $v0, 1               #   print int
move $a0, $t0               #   your input int
syscall 

li $v0, 4               #   print string
la $a0, nl              #       print out new line
syscall  

li $v0, 4               #   print string
la $a0, mesg3               #       print out the factorial is
syscall  

li $v0, 1               #   print int
move $a0, $t4               #   print factorial
syscall 

li $v0, 4               #   print string
la $a0, nl              #       print out new line
syscall 

li $v0, 4               #   print string
la $a0, mesg4               #       ask if they want to do it again
syscall 

li $v0, 8               #   user input 
la,$a0, userInput
li,$a1, 10
syscall

move,$t5,$v0
lw, $t6, yes
beq $t5,$t6,main            #   if yes, go  back to step 1
bne $t5,$t6,end             #   if no, terminate

Fact:
move $a0,$t0                # move user inputs into a0
subi $sp,$sp,8              # make room for 2 registers
sw $ra,4($sp) 
sw $a0,0($sp)
slti  $t0,$a0,1             # if (n<1)
beq $t0,$zero,L1
addi $v0,$zero,1            # return 1
addi $sp,$sp,8              # pop 2 from stack (not restored)
move $t4, $v0               # save return value into t4
jr $ra

L1:                     # n-1
subi $a0,$a0,1          
jal Fact                # recursion
lw $a0,0($sp)               # when we return back
lw $ra,4($sp)
addi $sp,$sp,8
mul $v0,$a0,$v0             # n*fact(n-1)
move $t4, $v0               # save return value into t4
jr $ra

error: 
li $v0, 4               #   print string
la $a0, mesg1               #       print out error message
syscall  

end: 
li $v0, 10              #   terminate program           
syscall  

How to modify it so it works but have to use the Fact: and L1: functions? I guess there's something wrong in the jr $ra? but dont know how to fix it...