Pfthb

I'm asking a user for input and I want to replace each character of the word by the number of uppercase characters in this word

Example:
Input: AaAA
output: 3333 (4 letters - 3 uppercase)

The part for counting upercase letters in a word works (stored in $t5)

However I cannot replace e.g "3333" with "AaAA". I'm trying to do it using sb in word_replace section. In my output I can see boxes instead of numbers.

Here is my code:

.data
prompt: .asciiz "Enter a string: "
msgout: .asciiz "Output string: "
input: .space 256
output: .space 256
 .text
 .globl main

main:
 li $v0, 4 # Print enter a string prompt
 la $a0, prompt 
 syscall

 li $v0, 8 # Ask the user for the string they want to reverse
 la $a0, input # We'll store it in 'input'
 li $a1, 256 # Only 256 chars/bytes allowed
 syscall

 la $t2, ($a0) # t2 - input string

 word:
 li $t1, 0 # Normal counter
 li $t5, 0 # Uppercase counter

 word_countUppercase:
 add $t3, $t2, $t1 # $t2 is the base address for our 'input' array, add loop index
 lb $t4, 0($t3) # load a byte at a time according to counter 

 bltu $t4, ' ', word_prereplace # We found end of word

 addi $t1, $t1, 1 # Advance our counter (i++)

 bltu $t4, 'A', word_countUppercase
 bgtu $t4, 'Z', word_countUppercase

 addi $t5, $t5, 1 # Advance our counter (i++)
 j word_countUppercase

 word_prereplace:
 la $t2, ($a0) # t2 - input string
 li $t1, 0 # Normal counter

 word_replace:
 add $t3, $t2, $t1 # $t2 is the base address for our 'input' array, add loop index
 lb $t4, 0($t3) # load a byte at a time according to counter 

 bltu $t4, ' ', exit # We found end of word

 sb $t5, output($t1) # Overwrite this byte address in memory 

 addi $t1, $t1, 1 # Advance our counter (i++)
 j word_replace


exit:
 li $v0, 4 # Print msgout
 la $a0, msgout
 syscall

 li $v0, 4 # Print the output string!
 la $a0, output
 syscall

 li $v0, 10 # exit()
 syscall