Why does "const extern" give an error?

The following piece of code working fine:

#include <stdio.h>

extern int foo; // Without constant
int foo = 42;

int main() 
{
    printf("%d\n",foo);
    return 0;
}

But, the following piece of code give an error:

#include <stdio.h>

const extern int foo; // With constant
int foo = 42;

int main() 
{
    printf("%d\n",foo);
    return 0;
}

So, why does const extern give an error?