Why fun(k++) is wrong, but fun(++k) is ok?

Question

Please tell me why fun(k++) is wrong while fun(++k) is okay in the following code. The error information is error: cannot bind non-const lvalue reference of type 'int&' to an rvalue of type 'int'

#include <iostream>

using std::cout;

void fun(int &k)
{
    cout << k;
}

int main()
{
    int k = 1;
    fun(k++); // error
    fun(++k); // good

    return 0;
}