undefined symbol : rcx, xor, rcx

.686P
.MODEL FLAT, C

.DATA
myArray DWORD 15, -10, 10, -20, 20, -30, 30, -40, 40, 100

arraySize DWORD ($ - myArray) / 8 

result DWORD 0  

.CODE

some PROC

    xor rcx, rcx   
    mov rax, 0   
while_start:

    cmp rcx, arraySize   
    jnl while_end  
    imul rdx, rcx, 8  
    mov rbx, myArray[rdx]  
    add rax, rbx  
    inc rcx   
    jmp while_start  
while_end:

    ret
some ENDP

END some