diff --git a/core/keyboard_nav/cursor.js b/core/keyboard_nav/cursor.js
index 01ed5b737..be9e2d48c 100644
--- a/core/keyboard_nav/cursor.js
+++ b/core/keyboard_nav/cursor.js
@@ -31,7 +31,7 @@ goog.provide('Blockly.Cursor');
  * @constructor
  */
 Blockly.Cursor = function() {
-  /*
+  /**
    * The current location of the cursor.
    * @type {Blockly.ASTNode}
    * @private
diff --git a/core/ui_menu_utils.js b/core/ui_menu_utils.js
index 35907e381..bd77bf750 100644
--- a/core/ui_menu_utils.js
+++ b/core/ui_menu_utils.js
@@ -39,7 +39,7 @@ goog.require('Blockly.utils.style');
  */
 Blockly.utils.uiMenu.getSize = function(menu) {
   var menuDom = menu.getElement();
-  var menuSize = Blockly.utils.style.getSize(menuDom);
+  var menuSize = Blockly.utils.style.getSize(/** @type {!Element} */ (menuDom));
   // Recalculate height for the total content, not only box height.
   menuSize.height = menuDom.scrollHeight;
   return menuSize;
diff --git a/core/utils.js b/core/utils.js
index 48f91e64e..f633f6c35 100644
--- a/core/utils.js
+++ b/core/utils.js
@@ -121,7 +121,7 @@ Blockly.utils.getInjectionDivXY_ = function(element) {
     if ((' ' + classes + ' ').indexOf(' injectionDiv ') != -1) {
       break;
     }
-    element = element.parentNode;
+    element = /** @type {!Element} */ (element.parentNode);
   }
   return new Blockly.utils.Coordinate(x, y);
 };
@@ -190,6 +190,7 @@ Blockly.utils.mouseToSvg = function(e, svg, matrix) {
 Blockly.utils.getScrollDeltaPixels = function(e) {
   switch (e.deltaMode) {
     case 0x00:  // Pixel mode.
+    default:
       return {
         x: e.deltaX,
         y: e.deltaY
@@ -236,7 +237,7 @@ Blockly.utils.replaceMessageReferences = function(message) {
   var interpolatedResult = Blockly.utils.tokenizeInterpolation_(message, false);
   // When parseInterpolationTokens == false, interpolatedResult should be at
   // most length 1.
-  return interpolatedResult.length ? interpolatedResult[0] : '';
+  return interpolatedResult.length ? String(interpolatedResult[0]) : '';
 };
 
 /**
diff --git a/core/utils/colour.js b/core/utils/colour.js
index 849737448..928fc935b 100644
--- a/core/utils/colour.js
+++ b/core/utils/colour.js
@@ -38,8 +38,8 @@ goog.require('Blockly.utils');
  * .parse('#f00') -> '#ff0000'
  * .parse('#ff0000') -> '#ff0000'
  * .parse('rgb(255, 0, 0)') -> '#ff0000'
- * @param {string} str Colour in some CSS format.
- * @return {string|null} A string containing a hex representation of the colour,
+ * @param {string|number} str Colour in some CSS format.
+ * @return {?string} A string containing a hex representation of the colour,
  *   or null if can't be parsed.
  */
 Blockly.utils.colour.parse = function(str) {
@@ -166,11 +166,19 @@ Blockly.utils.colour.hsvToHex = function(h, s, v) {
  * @param {string} colour2 Second colour.
  * @param {number} factor The weight to be given to colour1 over colour2.
  *     Values should be in the range [0, 1].
- * @return {string} Combined colour represented in hex.
+ * @return {?string} Combined colour represented in hex.
  */
 Blockly.utils.colour.blend = function(colour1, colour2, factor) {
-  var rgb1 = Blockly.utils.colour.hexToRgb(Blockly.utils.colour.parse(colour1));
-  var rgb2 = Blockly.utils.colour.hexToRgb(Blockly.utils.colour.parse(colour2));
+  var hex1 = Blockly.utils.colour.parse(colour1);
+  if (!hex1) {
+    return null;
+  }
+  var hex2 = Blockly.utils.colour.parse(colour2);
+  if (!hex2) {
+    return null;
+  }
+  var rgb1 = Blockly.utils.colour.hexToRgb(hex1);
+  var rgb2 = Blockly.utils.colour.hexToRgb(hex2);
   var r = Math.round(rgb2[0] + factor * (rgb1[0] - rgb2[0]));
   var g = Math.round(rgb2[1] + factor * (rgb1[1] - rgb2[1]));
   var b = Math.round(rgb2[2] + factor * (rgb1[2] - rgb2[2]));