Line 437 looks like this:

                        chars[0] += string_length + 1;

I understand that:

• chars[0] is a null pointer,
• string_length + 1 is the "non-zero offset 8".

Not sure I understand everything at this point, I need to run json-test in a debugger to grasp what this is about.

Also, how to reproduce the error you are referring to?

OK, I think I am able to reproduce it by running test_json on tests/valid-0003.json. Adding

                        printf("%p += %u\n", chars[0], string_length + 1);

to the source code yields:

(nil) += 4
0x4 += 4

But then, why would this necessarily be an actual error?

But then, why would this necessarily be an actual error?

Adding to a null pointer is undefined behavior. Its good style to not go off standard.

Ah right. I guess the code will have to be changed to:

chars[0] = (void*) ((uintptr_t) chars[0] + string_length + 1)