the value to "out" parameter must be assigned in the method body, otherwise the method will not be compiled

the value to "out" parameter must be assigned in the method body,
otherwise the method will not be compiled

I have the following code in c#
class Sample{
public void sum(out int num1,int num2)
{
}
public void print()
{ int i=12;
sum(out i,10);
Console.WriteLine(i);
}
}
why the following code give error saying "the out parameter 'num1' must be
assigned before control leaves the current method",even i am not writing
any statement there or not using num1 and i already assign it the value in
callee method?