Illegal hexadecimal number. Explanation: The format of a hexadecimal number is X'ffff', where ffff may be up to 16 hexadecimal digits. A hexadecimal digit is a numeric digit (0 to 9) or an A, B, C, D, E, or F. The letters may be upper or lower case. There must be an even number of digits. User Action: Enter a proper hexadecimal number. Make sure that the trailing apostrophe is included.