diff --git a/types/serialport/index.d.ts b/types/serialport/index.d.ts
index b0a374efeb..1e8aa9faa5 100644
--- a/types/serialport/index.d.ts
+++ b/types/serialport/index.d.ts
@@ -36,7 +36,7 @@ declare class SerialPort extends Stream.Duplex {
 	pause(): this;
 	resume(): this;
 
-	on(event: string, callback?: (data?: any) => void): this;
+	on(event: string, callback: (data?: any) => void): this;
 
 	static Binding: SerialPort.BaseBinding;