As Wikipedia for UTF-8 states:

It was designed for backward compatibility with ASCII: the first 128 characters of Unicode, which correspond one-to-one with ASCII, are encoded using a single byte with the same binary value as ASCII, so that a UTF-8-encoded file using only those characters is identical to an ASCII file.

Basically, ASCII is a subset of UTF-8 so the encoding just fails if a byte representation of your UTF-8 character is longer than 1 byte.