private function call( $resource, $data, $method = null ) {

		$method = is_null( $method ) ? $this->default_request_method : $method;

		// setup headers.
		$content_type = $this->is_json ? 'application/json; charset=utf-8' : 'application/x-www-form-urlencoded';
		$headers      = $this->set_request_headers(
			array(
				'content-type' => $content_type,
			),
			$resource,
			$method
		);

		$args = array(
			'headers'    => $headers,
			'method'     => $method,
			'timeout'    => $this->request_timeout,
			'user-agent' => $this->set_user_agent( 'LifterLMS ' . LLMS_VERSION, $resource, $method ),
		);

		// setup body.
		$body = $this->set_request_body( $data, $method, $resource );

		// if "null" if passed to body, don't send a body at all.
		if ( ! is_null( $body ) ) {
			$args['body'] = $this->is_json && $body ? wp_json_encode( $body ) : $body;
		}

		// Attempt to call the API.
		$response = wp_safe_remote_request(
			$this->set_request_url( $resource, $method ),
			$args
		);

		// Connection error.
		if ( is_wp_error( $response ) ) {
			return $this->set_error( __( 'There was a problem connecting to the external API.', 'lifterlms' ), 'api_connection', $response );
		}

		// Empty body.
		if ( ! $this->allow_empty_response && empty( $response['body'] ) ) {

			return $this->set_error( __( 'Empty Response.', 'lifterlms' ), 'empty_response', $response );

		}

		$this->parse_response( $response );

	}