diff --git a/blocks/logic.js b/blocks/logic.js
index 6b789db0a..15b08170f 100644
--- a/blocks/logic.js
+++ b/blocks/logic.js
@@ -542,14 +542,16 @@ Blockly.Constants.Logic.LOGIC_COMPARE_ONCHANGE_MIXIN = {
       var prevA = this.prevBlocks_[0];
       if (prevA !== blockA) {
         blockA.unplug();
-        if (prevA) {
+        if (prevA && !prevA.isShadow()) {
+          // The shadow block is automatically replaced during unplug().
           this.getInput('A').connection.connect(prevA.outputConnection);
         }
       }
       var prevB = this.prevBlocks_[1];
       if (prevB !== blockB) {
         blockB.unplug();
-        if (prevB) {
+        if (prevB && !prevB.isShadow()) {
+          // The shadow block is automatically replaced during unplug().
           this.getInput('B').connection.connect(prevB.outputConnection);
         }
       }