diff --git a/blocks/procedures.js b/blocks/procedures.js
index c74c15f6a..cae8b78ba 100644
--- a/blocks/procedures.js
+++ b/blocks/procedures.js
@@ -504,10 +504,16 @@ Blockly.Blocks['procedures_callnoreturn'] = {
     //     Existing param IDs.
     // Note that quarkConnections_ may include IDs that no longer exist, but
     // which might reappear if a param is reattached in the mutator.
-    if (!paramIds) {
-      // Reset the quarks (a mutator is about to open).
+    var defBlock = Blockly.Procedures.getDefinition(this.getProcedureCall(),
+        this.workspace);
+    var mutatorOpen = defBlock && defBlock.mutator &&
+        defBlock.mutator.isVisible();
+    if (!mutatorOpen) {
       this.quarkConnections_ = {};
       this.quarkIds_ = null;
+    }
+    if (!paramIds) {
+      // Reset the quarks (a mutator is about to open).
       return;
     }
     if (goog.array.equals(this.arguments_, paramNames)) {
@@ -539,7 +545,8 @@ Blockly.Blocks['procedures_callnoreturn'] = {
       if (input) {
         var connection = input.connection.targetConnection;
         this.quarkConnections_[this.quarkIds_[i]] = connection;
-        if (connection && paramIds.indexOf(this.quarkIds_[i]) == -1) {
+        if (mutatorOpen && connection &&
+            paramIds.indexOf(this.quarkIds_[i]) == -1) {
           // This connection should no longer be attached to this block.
           connection.disconnect();
           connection.getSourceBlock().bumpNeighbours_();
diff --git a/core/connection.js b/core/connection.js
index eb2749218..14c07b36f 100644
--- a/core/connection.js
+++ b/core/connection.js
@@ -331,7 +331,8 @@ Blockly.Connection.prototype.checkConnection_ = function(target) {
     case Blockly.Connection.REASON_SELF_CONNECTION:
       throw 'Attempted to connect a block to itself.';
     case Blockly.Connection.REASON_DIFFERENT_WORKSPACES:
-      throw 'Blocks are on different workspaces.';
+      // Usually this means one block has been deleted.
+      throw 'Blocks not on same workspace.';
     case Blockly.Connection.REASON_WRONG_TYPE:
       throw 'Attempt to connect incompatible types.';
     case Blockly.Connection.REASON_TARGET_NULL:
diff --git a/core/events.js b/core/events.js
index 9cf8ee901..12179eb30 100644
--- a/core/events.js
+++ b/core/events.js
@@ -423,6 +423,7 @@ Blockly.Events.Change.prototype.run = function(forward) {
         oldMutation = oldMutationDom && Blockly.Xml.domToText(oldMutationDom);
       }
       if (block.domToMutation) {
+        value = value || '<mutation></mutation>';
         var dom = Blockly.Xml.textToDom('<xml>' + value + '</xml>');
         block.domToMutation(dom.firstChild);
       }
diff --git a/core/mutator.js b/core/mutator.js
index 40f9221ec..91ef5814a 100644
--- a/core/mutator.js
+++ b/core/mutator.js
@@ -349,8 +349,8 @@ Blockly.Mutator.prototype.dispose = function() {
  * @return {boolean} True iff a reconnection was made, false otherwise.
  */
 Blockly.Mutator.reconnect = function(connectionChild, block, inputName) {
-  if (!connectionChild) {
-    return false;
+  if (!connectionChild || !connectionChild.getSourceBlock().workspace) {
+    return false;  // No connection or block has been deleted.
   }
   var connectionParent = block.getInput(inputName).connection;
   var currentParent = connectionChild.targetBlock();