Is it legal to pass a non-null-terminated string to strncmp in C?

According to the C99 standard, section 7.21.4.4, §3., it is legal:

The strncmp function returns an integer greater than, equal to, or less than zero, accordingly as the possibly null-terminated array pointed to by s1 is greater than, equal to, or less than the possibly null-terminated array pointed to by s2.

Notice, however, that it says array of characters. By definition, if an array of characters is not null-terminated, it is not a string.