MIPs code programming?

am working on a mips program to convert numbers from ascii string into integers, with the parameter that it should accept positive and negative integer decimal strings. I know this is a pretty common a assignment but I wasn't able to resolve my problems online. I was able to get some tips, but still receiving errors when trying to run the program. I haven't been able to fix the errors, I have no idea what to do with them. I am getting bad address exceptions starting with this line "lb $t1, ($t0) # load $t1 = digit character"

This is my first time coding in assembly, please give me some guidance, I am completely lost.

Here is the code I have at this time:

.data

string: .byte '1','2','3','0' # create a 2-element character array

.text

main: # convert string to integer

lw $a0, string

li $t6, 0x00 # $t6 = 0

li $t7, 0x09 # $t7 = 9

li $v0, 0x00 # initialize $v0 = 0

lb $t0, ($a0) # $t0 = pointer to string

lb $t1, ($t0) # load $t1 = digit character

loop:

blt $t1, $t6, Done # check for each char < 0

bgt $t1, $t7, Done # check for each char > 9

subu $t1, $t1, $t6 # convert char to int

mul $v0, $v0, 10 # mult by 10 to shift value

add $v0, $v0, $t1 # add new digit to value

addiu $t0, $t0, 1 # point to next char

lb $t1, ($t0) # load next digit

bne $t1, $0, loop # repeat if not end of string

jr $ra # return integer

Done: #exit program if string not a number

li $v0, -1 # return -1 in $v0

jr $ra