diff --git a/src/blocks/scratch3_pen.js b/src/blocks/scratch3_pen.js
index 2a0bd4908..407215120 100644
--- a/src/blocks/scratch3_pen.js
+++ b/src/blocks/scratch3_pen.js
@@ -452,7 +452,7 @@ class Scratch3PenBlocks {
         penState.saturation = hsv.s * 100;
         penState.brightness = hsv.v * 100;
         if (rgb.hasOwnProperty('a')) {
-            penState.transparency = 100 * (1 - rgb.a / 255.0);
+            penState.transparency = 100 * (1 - (rgb.a / 255.0));
         } else {
             penState.transparency = 0;
         }