Your irregular reminder: something referred to as a 'number' in a particular domain — account number, card number, phone number, ISBN, PIN, etc. — is rarely a number from a coding or mathematical perspective. Often 'numbers' contain letters, punctuation or other significant formatting. Even if they're just sequences of digits, they are just that, and a sequence of digits is not necessarily a mathematical number — if you're storing them as integers, now you know what you're fixing next week 🙃