diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index c276c57..abfc9ac 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -978,7 +978,7 @@ function theme($hook, $variables = array()) {
     $variables = array();
     if (isset($info['variables'])) {
       foreach (array_keys($info['variables']) as $name) {
-        if (isset($element["#$name"])) {
+        if (array_key_exists("#$name", $element)) {
           $variables[$name] = $element["#$name"];
         }
       }
diff --git a/core/modules/image/image.field.inc b/core/modules/image/image.field.inc
index 1f0e87b..cc7cc99 100644
--- a/core/modules/image/image.field.inc
+++ b/core/modules/image/image.field.inc
@@ -441,7 +441,7 @@ function theme_image_formatter($variables) {
   }
 
   foreach (array('width', 'height', 'alt', 'attributes') as $key) {
-    if (isset($item[$key])) {
+    if (array_key_exists($key, $item)) {
       $image["#$key"] = $item[$key];
     }
   }
diff --git a/core/modules/image/image.module b/core/modules/image/image.module
index c1918eb..9e12ab1 100644
--- a/core/modules/image/image.module
+++ b/core/modules/image/image.module
@@ -150,6 +150,18 @@ function image_theme() {
   return array(
     // Theme functions in image.module.
     'image_style' => array(
+      // HTML 4 and XHTML 1.0 always require an alt attribute. The HTML 5 draft
+      // allows the alt attribute to be omitted in some cases. Therefore,
+      // default the alt attribute to an empty string, but allow code calling
+      // theme('image_style') to pass explicit NULL for it to be omitted.
+      //  Usually, neither omission nor an empty string satisfies accessibility
+      // requirements, so it is strongly encouraged for code calling
+      // theme('image_style') to pass a meaningful value for the alt variable.
+      // - http://www.w3.org/TR/REC-html40/struct/objects.html#h-13.8
+      // - http://www.w3.org/TR/xhtml1/dtds.html
+      // - http://dev.w3.org/html5/spec/Overview.html#alt
+      // The title attribute is optional in all cases, so it is omitted by
+      // default.
       'variables' => array(
         'style_name' => NULL,
         'uri' => NULL,
@@ -362,7 +374,16 @@ function image_style_options($include_empty = TRUE) {
  *     'public://images/image.jpg'.
  *   - width: The width of the source image (if known).
  *   - height: The height of the source image (if known).
- *   - alt: The alternative text for text-based browsers.
+ *   - alt: The alternative text for text-based browsers. HTML 4 and XHTML 1.0
+ *     always require an alt attribute. The HTML 5 draft allows the alt
+ *     attribute to be omitted in some cases. Therefore, this variable defaults
+ *     to an empty string, but can be set to NULL for the attribute to be
+ *     omitted. Usually, neither omission nor an empty string satisfies
+ *     accessibility requirements, so it is strongly encouraged for code calling
+ *     theme('image_style') to pass a meaningful value for this variable.
+ *     - http://www.w3.org/TR/REC-html40/struct/objects.html#h-13.8
+ *     - http://www.w3.org/TR/xhtml1/dtds.html
+ *     - http://dev.w3.org/html5/spec/Overview.html#alt
  *   - title: The title text is displayed when the image is hovered in some
  *     popular browsers.
  *   - attributes: Associative array of attributes to be placed in the img tag.
@@ -393,10 +414,10 @@ function theme_image_style($variables) {
     '#uri' => $style->buildUrl($variables['uri']),
   );
 
-  if (isset($variables['alt'])) {
+  if (array_key_exists('alt', $variables)) {
     $image['#alt'] = $variables['alt'];
   }
-  if (isset($variables['title'])) {
+  if (array_key_exists('title', $variables)) {
     $image['#title'] = $variables['title'];
   }
 
diff --git a/core/modules/image/lib/Drupal/image/Tests/ImageThemeFunctionTest.php b/core/modules/image/lib/Drupal/image/Tests/ImageThemeFunctionTest.php
index f0d540f..1f00c21 100644
--- a/core/modules/image/lib/Drupal/image/Tests/ImageThemeFunctionTest.php
+++ b/core/modules/image/lib/Drupal/image/Tests/ImageThemeFunctionTest.php
@@ -43,20 +43,27 @@ function testImageFormatterTheme() {
     $style->save();
     $url = $style->buildUrl($original_uri);
 
-    // Test using theme_image_formatter() without an image title, alt text, or
-    // link options.
+    // Test using theme_image_formatter() with a NULL value for the alt option.
     $path = $this->randomName();
     $element = array(
       '#theme' => 'image_formatter',
       '#image_style' => 'test',
       '#item' => array(
         'uri' => $original_uri,
+        'alt' => NULL,
       ),
       '#path' => array(
         'path' => $path,
       ),
     );
     $rendered_element = render($element);
+    $expected_result = '<a href="' . base_path() . $path . '"><img class="image-style-test" src="' . $url . '" /></a>';
+    $this->assertEqual($expected_result, $rendered_element, 'theme_image_formatter() correctly renders with a NULL value for the alt option.');
+
+    // Test using theme_image_formatter() without an image title, alt text, or
+    // link options.
+    unset($element['#item']['alt']);
+    $rendered_element = render($element);
     $expected_result = '<a href="' . base_path() . $path . '"><img class="image-style-test" src="' . $url . '" alt="" /></a>';
     $this->assertEqual($expected_result, $rendered_element, 'theme_image_formatter() correctly renders without title, alt, or path options.');
 
@@ -95,6 +102,12 @@ function testImageStyleTheme() {
     $rendered_element = render($element);
     $expected_result = '<img class="image-style-image-test" src="' . $url . '" alt="" />';
     $this->assertEqual($expected_result, $rendered_element, 'theme_image_style() renders an image correctly.');
+
+    // Test using theme_image_style() with a NULL value for the alt option.
+    $element['#alt'] = NULL;
+    $rendered_element = render($element);
+    $expected_result = '<img class="image-style-image-test" src="' . $url . '" />';
+    $this->assertEqual($expected_result, $rendered_element, 'theme_image_style() renders an image correctly with a NULL value for the alt option.');
   }
 
 }