diff --git a/core/block.js b/core/block.js
index 18ff384cf..4ad3117b0 100644
--- a/core/block.js
+++ b/core/block.js
@@ -1150,13 +1150,10 @@ Blockly.Block.prototype.interpolate_ = function(message, args, lastDummyAlign) {
               input = this.appendDummyInput(element['name']);
               break;
             case 'field_label':
-              field = new Blockly.FieldLabel(element['text'], element['class']);
+              field = Blockly.Block.newFieldLabelFromJson_(element);
               break;
             case 'field_input':
-              field = new Blockly.FieldTextInput(element['text']);
-              if (typeof element['spellcheck'] == 'boolean') {
-                field.setSpellcheck(element['spellcheck']);
-              }
+              field = Blockly.Block.newFieldTextInputFromJson_(element);
               break;
             case 'field_angle':
               field = new Blockly.FieldAngle(element['angle']);
@@ -1169,14 +1166,13 @@ Blockly.Block.prototype.interpolate_ = function(message, args, lastDummyAlign) {
               field = new Blockly.FieldColour(element['colour']);
               break;
             case 'field_variable':
-              field = new Blockly.FieldVariable(element['variable']);
+              field = Blockly.Block.newFieldVariableFromJson_(element);
               break;
             case 'field_dropdown':
               field = new Blockly.FieldDropdown(element['options']);
               break;
             case 'field_image':
-              field = new Blockly.FieldImage(element['src'],
-                  element['width'], element['height'], element['alt']);
+              field = Blockly.Block.newFieldImageFromJson_(element);
               break;
             case 'field_number':
               field = new Blockly.FieldNumber(element['value'],
@@ -1215,6 +1211,64 @@ Blockly.Block.prototype.interpolate_ = function(message, args, lastDummyAlign) {
   }
 };
 
+/**
+ * Helper function to construct a FieldImage from a JSON arg object,
+ * dereferencing any string table references.
+ * @param {!Object} options A JSON object with options (src, width, height, and alt).
+ * @returns {!Blockly.FieldImage} The new image.
+ * @private
+ */
+Blockly.Block.newFieldImageFromJson_ = function(options) {
+  var src = Blockly.utils.replaceMessageReferences(options['src']);
+  var width = Number(Blockly.utils.replaceMessageReferences(options['width']));
+  var height =
+    Number(Blockly.utils.replaceMessageReferences(options['height']));
+  var alt = Blockly.utils.replaceMessageReferences(options['alt']);
+  return new Blockly.FieldImage(src, width, height, alt);
+};
+
+/**
+ * Helper function to construct a FieldLabel from a JSON arg object,
+ * dereferencing any string table references.
+ * @param {!Object} options A JSON object with options (text, and class).
+ * @returns {!Blockly.FieldImage} The new image.
+ * @private
+ */
+Blockly.Block.newFieldLabelFromJson_ = function(options) {
+  var text = Blockly.utils.replaceMessageReferences(options['text']);
+  return new Blockly.FieldLabel(text, options['class']);
+};
+
+/**
+ * Helper function to construct a FieldTextInput from a JSON arg object,
+ * dereferencing any string table references.
+ * @param {!Object} options A JSON object with options (text, class, and
+ *                          spellcheck).
+ * @returns {!Blockly.FieldImage} The new image.
+ * @private
+ */
+Blockly.Block.newFieldTextInputFromJson_ = function(options) {
+  var text = Blockly.utils.replaceMessageReferences(options['text']);
+  var field = new Blockly.FieldTextInput(text, options['class']);
+  if (typeof options['spellcheck'] == 'boolean') {
+    field.setSpellcheck(options['spellcheck']);
+  }
+  return field;
+};
+
+/**
+ * Helper function to construct a FieldVariable from a JSON arg object,
+ * dereferencing any string table references.
+ * @param {!Object} options A JSON object with options (variable).
+ * @returns {!Blockly.FieldImage} The new image.
+ * @private
+ */
+Blockly.Block.newFieldVariableFromJson_ = function(options) {
+  var varname = Blockly.utils.replaceMessageReferences(options['variable']);
+  return new Blockly.FieldVariable(varname);
+};
+
+
 /**
  * Add a value input, statement input or local variable to this block.
  * @param {number} type Either Blockly.INPUT_VALUE or Blockly.NEXT_STATEMENT or
diff --git a/core/field_image.js b/core/field_image.js
index e91f00070..028905922 100644
--- a/core/field_image.js
+++ b/core/field_image.js
@@ -35,8 +35,8 @@ goog.require('goog.userAgent');
 /**
  * Class for an image on a block.
  * @param {string} src The URL of the image.
- * @param {number|string} width Width of the image, possibly via reference.
- * @param {number|string} height Height of the image, possibly via reference.
+ * @param {number} width Width of the image.
+ * @param {number} height Height of the image.
  * @param {string=} opt_alt Optional alt text for when block is collapsed.
  * @extends {Blockly.Field}
  * @constructor
@@ -44,18 +44,6 @@ goog.require('goog.userAgent');
 Blockly.FieldImage = function(src, width, height, opt_alt) {
   this.sourceBlock_ = null;
 
-  // Replace any message references in the arguments.
-  src = Blockly.utils.replaceMessageReferences(src);
-  if (goog.isString(height)) {
-    height = Blockly.utils.replaceMessageReferences(height);
-  }
-  if (goog.isString(width)) {
-    width = Blockly.utils.replaceMessageReferences(width);
-  }
-  if (goog.isString(opt_alt)) {
-    opt_alt = Blockly.utils.replaceMessageReferences(opt_alt);
-  }
-
   // Ensure height and width are numbers.  Strings are bad at math.
   this.height_ = Number(height);
   this.width_ = Number(width);
diff --git a/core/utils.js b/core/utils.js
index 5066c34f4..b4928044e 100644
--- a/core/utils.js
+++ b/core/utils.js
@@ -259,7 +259,7 @@ Blockly.utils.getRelativeXY.XY_2D_REGEX_ =
  *     context (scale...).
  * @return {!SVGElement} Newly created SVG element.
  */
-Blockly.utils.createSvgElement = function(name, attrs, parent, opt_workspace) {
+Blockly.utils.createSvgElement = function(name, attrs, parent /*, opt_workspace */) {
   var e = /** @type {!SVGElement} */ (
       document.createElementNS(Blockly.SVG_NS, name));
   for (var key in attrs) {
@@ -409,13 +409,17 @@ Blockly.utils.tokenizeInterpolation = function(message) {
 };
 
 /**
- * Replaces string table references in a message string. For example,
- * %{bky_my_msg} and %{BKY_MY_MSG} will both be replaced with the value in
- * Blockly.Msg['MY_MSG'].
- * @param {string} message Text which might contain string table references.
+ * Replaces string table references in a message, if the message is a string.
+ * For example, "%{bky_my_msg}" and "%{BKY_MY_MSG}" will both be replaced with
+ * the value in Blockly.Msg['MY_MSG'].
+ * @param {string|?} message Message, which may be a string that contains
+ *                           string table references.
  * @return {!string} String with message references replaced.
  */
 Blockly.utils.replaceMessageReferences = function(message) {
+  if (!goog.isString(message)) {
+    return message;
+  }
   var interpolatedResult = Blockly.utils.tokenizeInterpolation_(message, false);
   // When parseInterpolationTokens == false, interpolatedResult should be at
   // most length 1.