What makes valid Base64?

A valid Base64 string contains only characters from the set A–Z, a–z, 0–9, +, and /, optionally padded with = at the end. The total length (without whitespace) must be divisible by 4.

Common issues

Missing padding: The string length mod 4 is not 0. Add = until it is. Invalid characters: Spaces, newlines, or non-Base64 characters are present. URL-safe chars: - and _ are not standard Base64 — they belong to Base64URL.