MIPS - Assembler - store address not aligned on word boundary

I'm trying to create BMP file from scratch in Mips (Mars 4.5 emulator). In order to do so, I'm trying to write its headers to some buffers. But saddly I get error of beeing "not aligned on word boundary" in first "save word" instruction. Buffer size does not affects this error :/ (BMP header has 14 bytes, I tried extending buffer to 16 bytes but no result).

Code:

la $s0, BMP_header
li $t0, 0x424D4600
li $t1, 0x00000000
li $t2, 0x00003600
li $t3, 0x00000000

sw $t0, ($s0)
addi $s0, $s0, 4 #BM, size
sw $t1, ($s0)
addi $s0, $s0, 4 #size, 00
sw $t2, ($s0)
addi $s0, $s0, 4 #00 offset
sw $t4, ($s0)
addi $s0, $s0, 4 #offset 00

It looks like I'm missing something that makes buffer alignment to correct address, but I have no clue what that is.