Without knowing the exact situation, if you follow the guidelines in this article, this is a library bug (documentation or actual code). Either the library sho

Without knowing the exact situation, if you follow the guidelines in this article, this is a library bug (documentation or actual code). Either the library should have enforced the invariant of the key existing (and returned an equivalent error, or handled it internally), or documented the preconditions at a higher level function that you could see.