Top Most Common Base64 Decoding Errors
1. "The input is not a valid Base-64 string" / FormatException
Cause: The input string contains illegal characters outside the Base64 alphabet (such as spaces, line breaks, or URL query parameters %20) or is missing required trailing = padding.
Solution: Use our Base64 Validator Tool to clean whitespace and strip invalid characters automatically.Base64 Validator Tool
2. Garbled / Cryptic Characters After Decoding (UTF-8 Corruptions)
Cause: Decoding UTF-8 multi-byte characters (emojis, accented characters, Asian script) using legacy ASCII or ISO-8859-1 decoders (such as native browser atob()).
Solution: Always decode multi-byte UTF-8 bytes using TextDecoder('utf-8') rather than simple ASCII string conversion.
3. Missing Equal Sign Padding Errors (Length % 4 != 0)
Cause: Many APIs and Base64URL implementations strip trailing = characters before transport.
Solution: Calculate str.length % 4 and append missing = characters until length is divisible by 4.