diff --git a/demos/custom-fields/blocks.js b/demos/custom-fields/blocks.js
index a49cd4352..298303812 100644
--- a/demos/custom-fields/blocks.js
+++ b/demos/custom-fields/blocks.js
@@ -51,9 +51,20 @@ Blockly.Blocks['turtle_nullifier'] = {
   },
 
   validate: function(newValue) {
+    this.cachedValidatedValue_ = Object.assign({}, newValue);
     if ((newValue.turtleName == 'Leonardo' && newValue.hat == 'Mask') ||
         (newValue.turtleName == 'Yertle' && newValue.hat == 'Crown') ||
         (newValue.turtleName == 'Franklin') && newValue.hat == 'Propeller') {
+
+      var currentValue = this.getValue();
+      if (newValue.turtleName != currentValue.turtleName) {
+        // Turtle name changed.
+        this.cachedValidatedValue_.turtleName = null;
+      } else {
+        // Hat must have changed.
+        this.cachedValidatedValue_.hat = null;
+      }
+
       return null;
     }
     return newValue;
diff --git a/demos/custom-fields/custom-fields.css b/demos/custom-fields/custom-fields.css
index af0ba506d..5162e0392 100644
--- a/demos/custom-fields/custom-fields.css
+++ b/demos/custom-fields/custom-fields.css
@@ -46,6 +46,7 @@
 }
 
 .customFieldsTurtleWidget .text {
+    height: 20px;
     color: #fff;
     flex-grow: 1;
     text-align: center;
diff --git a/demos/custom-fields/field_turtle.js b/demos/custom-fields/field_turtle.js
index 120a94608..adaa8954f 100644
--- a/demos/custom-fields/field_turtle.js
+++ b/demos/custom-fields/field_turtle.js
@@ -172,15 +172,16 @@ CustomFields.FieldTurtle.prototype.doClassValidation_ = function(newValue) {
     newValue.turtleName = null;
   }
 
+  // This is a strategy for dealing with defaults on multi-part values.
+  // The class validator sets individual properties of the object to null
+  // to indicate that they are invalid, and then caches that object to the
+  // cachedValidatedValue_ property. This way the field can, for
+  // example, properly handle an invalid pattern, combined with a valid hat.
+  // This can also be done with local validators.
+  this.cachedValidatedValue_ = newValue;
+
   // Always be sure to return!
   if (!newValue.pattern || !newValue.hat || !newValue.turtleName) {
-    // This is a strategy for dealing with defaults on multi-part values.
-    // The class validator sets individual properties of the object to null
-    // to indicate that they are invalid, and then caches that object to the
-    // cachedValidatedValue_ property. This way the field can, for
-    // example, properly handle an invalid pattern, combined with a valid hat.
-    // This can also be done with local validators.
-    this.cachedValidatedValue_ = newValue;
     return null;
   }
   return newValue;
@@ -221,9 +222,7 @@ CustomFields.FieldTurtle.prototype.render_ = function() {
   // Always do editor updates inside render. This makes sure the editor
   // always displays the correct value, even if a validator changes it.
   if (this.editor_) {
-    this.editor_.patternText.textContent = value.pattern;
-    this.editor_.hatText.textContent = value.hat;
-    this.editor_.turtleNameText.textContent = value.turtleName;
+    this.renderEditor_();
   }
 
   this.stovepipe_.style.display = 'none';
@@ -286,6 +285,23 @@ CustomFields.FieldTurtle.prototype.render_ = function() {
   this.updateSize_();
 };
 
+CustomFields.FieldTurtle.prototype.renderEditor_ = function() {
+  var value = this.displayValue_;
+
+  // .textElement is a property assigned to the element.
+  // It allows the text to be edited without destroying the warning icon.
+  this.editor_.patternText.textElement.nodeValue = value.pattern;
+  this.editor_.hatText.textElement.nodeValue = value.hat;
+  this.editor_.turtleNameText.textElement.nodeValue = value.turtleName;
+
+  this.editor_.patternText.warningIcon.style.display =
+    this.cachedValidatedValue_.pattern ? 'none' : '';
+  this.editor_.hatText.warningIcon.style.display =
+    this.cachedValidatedValue_.hat ? 'none' : '';
+  this.editor_.turtleNameText.warningIcon.style.display =
+    this.cachedValidatedValue_.turtleName ? 'none' : '';
+};
+
 // Used to update the size of the field. This function's logic could be simply
 // included inside render_ (it is not called anywhere else), but it is
 // usually separated to keep code more organized.
@@ -307,6 +323,7 @@ CustomFields.FieldTurtle.prototype.updateSize_ = function() {
 // this function to check/uncheck itself.
 CustomFields.FieldTurtle.prototype.showEditor_ = function() {
   this.editor_ = this.dropdownCreate_();
+  this.renderEditor_();
   Blockly.DropDownDiv.getContentDiv().appendChild(this.editor_);
 
   // These allow us to have the editor match the block's colour.
@@ -344,6 +361,14 @@ CustomFields.FieldTurtle.prototype.dropdownCreate_ = function() {
     cell.className = 'text';
     var text = document.createTextNode(text);
     cell.appendChild(text);
+    cell.textElement = text;
+    var warning = document.createElement('img');
+    warning.setAttribute('src', 'media/warning.svg');
+    warning.setAttribute('height', '16px');
+    warning.setAttribute('width', '16px');
+    warning.style.marginLeft = '4px';
+    cell.appendChild(warning);
+    cell.warningIcon = warning;
     row.appendChild(cell);
     return cell;
   };
diff --git a/demos/custom-fields/media/warning.svg b/demos/custom-fields/media/warning.svg
new file mode 100644
index 000000000..11136c1ef
--- /dev/null
+++ b/demos/custom-fields/media/warning.svg
@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16">
+    <path style="fill: #00c; stroke: #fff; stroke-width: 1px;" d="M2,15Q-1,15 0.5,12L6.5,1.7Q8,-1 9.5,1.7L15.5,12Q17,15 14,15z"></path>
+    <path style="fill: #fff;" d="m7,4.8v3.16l0.27,2.27h1.46l0.27,-2.27v-3.16z"></path>
+    <rect style="fill: #fff;" x="7" y="11" height="2" width="2"></rect>
+</svg>