diff --git a/lib/irc.js b/lib/irc.js
index fe85f0c..dafa99b 100644
--- a/lib/irc.js
+++ b/lib/irc.js
@@ -954,7 +954,7 @@ export function generateAuthenticateMessages(payload) {
 	// last chunk is exactly 400 bytes long
 	let msgs = [];
 	for (let i = 0; i <= encoded.length; i += maxSASLLength) {
-		let chunk = encoded.substring(i, i + 400);
+		let chunk = encoded.substring(i, i + maxSASLLength);
 		msgs.push({ command: "AUTHENTICATE", params: [chunk || "+"] });
 	}