diff --git a/core/modules/simpletest/files/image-test-transparent-out-of-range.gif b/core/modules/simpletest/files/image-test-transparent-out-of-range.gif
new file mode 100644
index 0000000000000000000000000000000000000000..a54df7a93ce2346e19414536a3a8eb555f64f8d1
--- /dev/null
+++ b/core/modules/simpletest/files/image-test-transparent-out-of-range.gif
@@ -0,0 +1,2 @@
+GIF89a(  Â      ÿ   ÿÿ  ÿÿ ÿÿÿ      !ù   ,    (   |8ºþ0ÊªlÍÍÝµ™ÒÔ‡…d
+˜gp,ÏEù…Tàl±¸Ïä{ Qºá§è1“¡žíò|ì¢Ä©…Àí^Þ®`L0er˜^§Çç·`ýÕÑåñ7½ÆGä{vnoyiV4‰0zk*$€Ž‘a“”–—r	 ;
\ No newline at end of file
diff --git a/core/modules/system/src/Plugin/ImageToolkit/GDToolkit.php b/core/modules/system/src/Plugin/ImageToolkit/GDToolkit.php
index 6234b6aee49bdb5580a4c87d77b41eeb50d06ab3..98ae86393ea2fb47af698c9cdf20e42f32ee64c4 100644
--- a/core/modules/system/src/Plugin/ImageToolkit/GDToolkit.php
+++ b/core/modules/system/src/Plugin/ImageToolkit/GDToolkit.php
@@ -239,7 +239,8 @@ public function createTmp($type, $width, $height) {
           // truecolor image or if the transparent color is part of the palette.
           // Since the index of the transparency color is a property of the
           // image rather than of the palette, it is possible that an image
-          // could be created with this index set outside the palette size.
+          // could be created with this index set outside the palette size (see
+          // http://stackoverflow.com/a/3898007).
           $transparent_color = imagecolorsforindex($this->getResource(), $transparent);
           $transparent = imagecolorallocate($res, $transparent_color['red'], $transparent_color['green'], $transparent_color['blue']);
 
diff --git a/core/modules/system/src/Tests/Image/ToolkitGdTest.php b/core/modules/system/src/Tests/Image/ToolkitGdTest.php
index f3869be3c46c7f246725ad17d6b8f2816d696c50..9288f5df0d7b261f7622338181f957e0ff3b1285 100644
--- a/core/modules/system/src/Tests/Image/ToolkitGdTest.php
+++ b/core/modules/system/src/Tests/Image/ToolkitGdTest.php
@@ -306,10 +306,39 @@ function testManipulations() {
 
         // Check that saved image reloads without raising PHP errors.
         $image_reloaded = $this->imageFactory->get($file_path);
+        $resource = $image_reloaded->getToolkit()->getResource();
       }
     }
   }
 
+  /**
+   * Tests loading an image whose transparent color index is out of range.
+   */
+  function testTransparentColorOutOfRange() {
+    // This image was generated by taking an initial image with a palette size
+    // of 6 colors, and setting the transparent color index to 6 (one higher
+    // than the largest allowed index), as follows:
+    // @code
+    // $image = imagecreatefromgif('core/modules/simpletest/files/image-test.gif');
+    // imagecolortransparent($image, 6);
+    // imagegif($image, 'core/modules/simpletest/files/image-test-transparent-out-of-range.gif');
+    // @endcode
+    // This allows us to test that an image with an out-of-range color index
+    // can be loaded correctly.
+    $file = 'image-test-transparent-out-of-range.gif';
+    $image = $this->imageFactory->get(drupal_get_path('module', 'simpletest') . '/files/' . $file);
+    $toolkit = $image->getToolkit();
+
+    if (!$image->isValid()) {
+      $this->fail(String::format('Could not load image %file.', array('%file' => $file)));
+    }
+    else {
+      // All images should be converted to truecolor when loaded.
+      $image_truecolor = imageistruecolor($toolkit->getResource());
+      $this->assertTrue($image_truecolor, String::format('Image %file after load is a truecolor image.', array('%file' => $file)));
+    }
+  }
+
   /**
    * Tests calling a missing image operation plugin.
    */