Yes, the spec says the field can be string or binary. The API only handles string fields. The API should be (and was) updated with a function that handles binary fields, but there's nothing wrong with the original function.

If this was C++, and the function took a std::string, would you say it was horribly broken because you serialized a 4 byte IP address into a 4 byte std::string buffer and the function didn't handle it correctly?