How do I find out why a "address out of range in MIPS", although temporary variables are declared

I am doing a homework problem for the computer design and architecture class where we are required to implement a simple for loop in C, into MIPS using the MARSim. I implemented the for loop step by step, initialized (if that is the proper term) the variables in memory, yet when I assemble and run it throws this error: question1.asm line 11: Runtime exception at 0x00400008: address out of range 0x00000000

I've looked at line 11 : lw $t1, 0($a1), and as I understand it, this should work properly. As I understand here we are setting our t1 value to a1 (b[i]).

Here is the C we have to reproduce:

for (i=0; i<=100; i++) { a[i] = b[i] + C ; }

Here is my attempt:

# t0 = i
# t1 = b[i]
# t2 = a[i]
# t3 = 101 (the end value of i)
# s0 = c
# $a0 = a
# $a1 = b
begin:
addi $t3, $zero, 101 #loop terminate value
add $t0, $zero, $zero # set our counter to zero
loop: lw $t1, 0($a1) # set t1 to b[i]
add $t1, $t1, $s0 # B[i] + c
sw $t1, 0($a0) #store $t1 into a[i]
addi $t0, $t0, 1  # loop increment
addi $a0,$a0,4 # increment a0 to point to the next block in the array (4 bits)
addi $a1, $a1, 4 # likewise with b[i]
beq $t0, $s0, finish    
finish:

The MIPS code I've written, should mimic the action of the C code w/o error. However on the first iteration of the loop it states it is accessing an out of range address 0x00000000. Could someone shine some light on what I am doing wrong? I would really appreciate a thorough explanation so I can understand this better for my class. Thanks for your assistance and much love. Cheers.