diff --git a/src/Terminal.ts b/src/Terminal.ts
index 36d9559e85..6675e4445d 100644
--- a/src/Terminal.ts
+++ b/src/Terminal.ts
@@ -984,7 +984,7 @@ export class Terminal extends EventEmitter implements ITerminal, IDisposable, II
           : 65;
           break;
         case 'wheel':
-          button = (<WheelEvent>ev).wheelDeltaY > 0
+          button = (<WheelEvent>ev).deltaY < 0
             ? 64
           : 65;
           break;