diff --git a/core/block_svg.js b/core/block_svg.js
index 1d1d57835..338db0d15 100644
--- a/core/block_svg.js
+++ b/core/block_svg.js
@@ -1123,6 +1123,11 @@ Blockly.BlockSvg.prototype.setMutator = function(mutator) {
     this.mutator = mutator;
     mutator.createIcon();
   }
+  if (this.rendered) {
+    this.render();
+    // Adding or removing a mutator icon will cause the block to change shape.
+    this.bumpNeighbours();
+  }
 };
 
 /**