diff --git a/blocks/logic.js b/blocks/logic.js
index 89f2b2097..813e8f747 100644
--- a/blocks/logic.js
+++ b/blocks/logic.js
@@ -561,7 +561,7 @@ Blockly.Constants.Logic.LOGIC_COMPARE_ONCHANGE_MIXIN = {
       var prevA = this.prevBlocks_[0];
       if (prevA !== blockA) {
         blockA.unplug();
-        if (prevA && !prevA.isShadow()) {
+        if (prevA && !prevA.isDisposed() && !prevA.isShadow()) {
           // The shadow block is automatically replaced during unplug().
           this.getInput('A').connection.connect(prevA.outputConnection);
         }
@@ -569,7 +569,7 @@ Blockly.Constants.Logic.LOGIC_COMPARE_ONCHANGE_MIXIN = {
       var prevB = this.prevBlocks_[1];
       if (prevB !== blockB) {
         blockB.unplug();
-        if (prevB && !prevB.isShadow()) {
+        if (prevB && !prevB.isDisposed() && !prevB.isShadow()) {
           // The shadow block is automatically replaced during unplug().
           this.getInput('B').connection.connect(prevB.outputConnection);
         }
diff --git a/core/block.js b/core/block.js
index 46893575a..fcb1da9c1 100644
--- a/core/block.js
+++ b/core/block.js
@@ -843,6 +843,14 @@ Blockly.Block.prototype.setEditable = function(editable) {
   }
 };
 
+/**
+ * Returns if this block has been disposed of / deleted.
+ * @return {boolean} True if this block has been disposed of / deleted.
+ */
+Blockly.Block.prototype.isDisposed = function() {
+  return this.disposed;
+};
+
 /**
  * Find the connection on this block that corresponds to the given connection
  * on the other block.