src/Message.php
private function setBody($body)
    {
        if (is_null($body)) {
            throw new InvalidArgumentException('Body of message cannot be null');
        }

        if (!is_string($body)) {
            throw new InvalidArgumentException(
                sprintf(
                    'Body of message must be a string, but "%s" given',
                    is_object($body) ? get_class($body) : gettype($body)
                )
            );
        }

        if (mb_strlen($body, 'UTF-8') === 0) {
            throw new InvalidArgumentException('Body of message cannot be an empty string');
        }

        $this->body = (string) $body;
    }
private function setBody($body)
    {
        if (is_null($body)) {
            throw new InvalidArgumentException('Body of message cannot be null');
        }

        if (!is_string($body)) {
            throw new InvalidArgumentException(
                sprintf(
                    'Body of message must be a string, but "%s" given',
                    is_object($body) ? get_class($body) : gettype($body)
                )
            );
        }

        if (mb_strlen($body, 'UTF-8') === 0) {
            throw new InvalidArgumentException('Body of message cannot be an empty string');
        }

        $this->body = (string) $body;
    }
/*
@param string $body