diff --git a/HISTORY.md b/HISTORY.md
index 945c7c01..6027e237 100755
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -23,6 +23,7 @@
 * objects: honor (hidden) blurred shadows setting for watcher cell widgets
 * tables: honor (hidden) blurred shadows setting for pictograms in tables
 * gui: made (hidden) "Blurred shadows" setting persistent, use to get rid of "red bar" artifacts on old laptops using Chrome
+* blocks: fixed a zoom blocks glitch
 
 ### 2020-10-05
 * Greek translation updata, thanks, HM100!
diff --git a/src/blocks.js b/src/blocks.js
index f3ae449d..0b583669 100644
--- a/src/blocks.js
+++ b/src/blocks.js
@@ -10889,6 +10889,10 @@ BooleanSlotMorph.prototype.drawKnob = function (ctx, progress) {
     ctx.arc(x, y, r - (outline / 2), radians(0), radians(360));
     ctx.stroke();
 
+    if (r < outline + this.edge) {
+        return;
+    }
+
     // top-left:
     gradient = ctx.createRadialGradient(
         x,