diff --git a/core/block.js b/core/block.js
index 50762ea05..5386b4372 100644
--- a/core/block.js
+++ b/core/block.js
@@ -751,7 +751,7 @@ Blockly.Block.prototype.getVars = function() {
   var vars = [];
   for (var i = 0, input; input = this.inputList[i]; i++) {
     for (var j = 0, field; field = input.fieldRow[j]; j++) {
-      if (field instanceof Blockly.FieldVariable) {
+      if (field.referencesVariables()) {
         vars.push(field.getValue());
       }
     }
@@ -768,7 +768,7 @@ Blockly.Block.prototype.getVarModels = function() {
   var vars = [];
   for (var i = 0, input; input = this.inputList[i]; i++) {
     for (var j = 0, field; field = input.fieldRow[j]; j++) {
-      if (field instanceof Blockly.FieldVariable) {
+      if (field.referencesVariables()) {
         var model = this.workspace.getVariableById(field.getValue());
         // Check if the variable actually exists (and isn't just a potential
         // variable).
@@ -790,7 +790,7 @@ Blockly.Block.prototype.getVarModels = function() {
 Blockly.Block.prototype.updateVarName = function(variable) {
   for (var i = 0, input; input = this.inputList[i]; i++) {
     for (var j = 0, field; field = input.fieldRow[j]; j++) {
-      if (field instanceof Blockly.FieldVariable &&
+      if (field.referencesVariables() &&
           variable.getId() == field.getValue()) {
         field.setText(variable.name);
       }
@@ -808,7 +808,7 @@ Blockly.Block.prototype.updateVarName = function(variable) {
 Blockly.Block.prototype.renameVarById = function(oldId, newId) {
   for (var i = 0, input; input = this.inputList[i]; i++) {
     for (var j = 0, field; field = input.fieldRow[j]; j++) {
-      if (field instanceof Blockly.FieldVariable &&
+      if (field.referencesVariables() &&
           oldId == field.getValue()) {
         field.setValue(newId);
       }
diff --git a/core/field.js b/core/field.js
index 3f915f2db..e90c2f1bb 100644
--- a/core/field.js
+++ b/core/field.js
@@ -604,3 +604,14 @@ Blockly.Field.prototype.setTooltip = function(_newTip) {
 Blockly.Field.prototype.getAbsoluteXY_ = function() {
   return goog.style.getPageOffset(this.borderRect_);
 };
+
+/**
+ * Whether this field references any Blockly variables.  If true it may need to
+ * be handled differently during serialization and deserialization.  Subclasses
+ * may override this.
+ * @return {boolean} True if this field has any variable references.
+ * @package
+ */
+Blockly.Field.prototype.referencesVariables = function() {
+  return false;
+};
diff --git a/core/field_variable.js b/core/field_variable.js
index 255c67a0c..3a97c4031 100644
--- a/core/field_variable.js
+++ b/core/field_variable.js
@@ -353,4 +353,14 @@ Blockly.FieldVariable.prototype.onItemSelected = function(menu, menuItem) {
   this.setValue(id);
 };
 
+/**
+ * Overrides referencesVariables(), indicating this field refers to a variable.
+ * @return {boolean} True.
+ * @package
+ * @override
+ */
+Blockly.FieldVariable.prototype.referencesVariables = function() {
+  return true;
+};
+
 Blockly.Field.register('field_variable', Blockly.FieldVariable);
diff --git a/core/xml.js b/core/xml.js
index 4f1507fff..15199c1ac 100644
--- a/core/xml.js
+++ b/core/xml.js
@@ -138,7 +138,7 @@ Blockly.Xml.fieldToDomVariable_ = function(field) {
  */
 Blockly.Xml.fieldToDom_ = function(field) {
   if (field.name && field.EDITABLE) {
-    if (field instanceof Blockly.FieldVariable) {
+    if (field.referencesVariables()) {
       return Blockly.Xml.fieldToDomVariable_(field);
     } else {
       var container = goog.dom.createDom('field', null, field.getValue());
@@ -806,7 +806,7 @@ Blockly.Xml.domToField_ = function(block, fieldName, xml) {
 
   var workspace = block.workspace;
   var text = xml.textContent;
-  if (field instanceof Blockly.FieldVariable) {
+  if (field.referencesVariables()) {
     Blockly.Xml.domToFieldVariable_(workspace, xml, text, field);
   } else {
     field.setValue(text);