Assembly behavior changed just because `xored` (when already xored)

Question

I'm standing in front of a weird situation: I have an assembly code that should return whether a number is prime or not. In the middle of the code, I have a register that is in zero state when arriving to it; But - the code return always 0 in EAX, unless I go to this register and explicitly put 0 in it -> Just then, and only then, the code return correctly 1/0 in EAX when asking it for a prime number. Here is a snippet of the code that does not work:

__asm__ (
    "IS_PRIME:;"
    "push EBP;"
    "MOV EBP, ESP;"


    "MOV ECX, 2;"
    "_LOOP_PRIME:;"


    "CALL IS_FACTOR;"

    "CMP EAX, 1;"
    "JZ FALSE_PRIME;"

    "INC ECX;"
    "CMP ECX, EBX;"

    "JNZ _LOOP_PRIME;"

    "MOV EAX, 1;"
    "JMP DONE;"

    "FALSE_PRIME:;"
    "MOV EAX, 0;"

    "DONE:;"
    "MOV ESP, EBP;"
    "POP EBP;"
    "JMP FINAL;"


    "IS_FACTOR:;"

    "MOV ESI, ECX;"
    "MOV EAX, EBX;"

    "DIV ESI;"

    "test EDX, EDX;"
    "JNZ FALSE;"


    "MOV EAX, 1;"
    "JMP DONE_IS_FACTOR;"
    "FALSE:;"
    "MOV EAX, 0;"

    "DONE_IS_FACTOR:;"
    "ret;"

    "FINAL:;"

);

And here is a snippet of replace when I the code do working:

"MOV ESI, ECX;"
"MOV EAX, EBX;"
"MOV EDX, 0;" # THIS IS THE INSTRUCTION I HAVE TO ADD FOR IT TO WORK.
"DIV ESI;"

My question is:
If anyway I get 0 in EDX when getting there in the first code, then why do I have to explicitly put 0 in there (as in the second snip) for the code to return correctly for whether it got a prime number in EBX or not?