undefined reference to array of constants

a.cpp

const unsigned char whatever[123] = { /* ... */ };

a.h

extern const unsigned char whatever[123];

b.cpp

#include "a.h"
unsigned char x = whatever[0];
// error: undefined reference to 'whatever'

Why do I get an undefined reference error? Without the const, the error goes away.

How do I share an array of constants among multiple translation units?