correctly pass parameters to export of a dll using rundll32

Question

Here is code of export function of a DLL that just adds two number and prints it to a MessageBox.

int Add(int a, int b)

{

OutputDebugString(L"add function called");

int sum = 0;
sum = a + b;

// MessageBoxA();

char buf[10];
_itoa(sum, buf, 10);
MessageBoxA(NULL, buf, "Caption", MB_OK);

When I pass the parameters to the exported function using rundll32 (command rundll32 dllName,Add 2 3), I get some junk number popped up in messagebox. But when I use Olldebugger to debug, I get the expected result which is the sum of numbers.

How do I correctly pass the parameters to the DLL export using rundll32?