diff --git a/modules/block/block.module b/modules/block/block.module
index 7790600..6bbc42e 100644
--- a/modules/block/block.module
+++ b/modules/block/block.module
@@ -943,7 +943,15 @@ function template_preprocess_block(&$variables) {
 
   $variables['theme_hook_suggestions'][] = 'block__' . $variables['block']->region;
   $variables['theme_hook_suggestions'][] = 'block__' . $variables['block']->module;
-  $variables['theme_hook_suggestions'][] = 'block__' . $variables['block']->module . '__' . $variables['block']->delta;
+  // Hyphens (-) and underscores (_) play a special role in theme suggestions.
+  // Theme suggestions should only contain underscores, because within
+  // drupal_find_theme_templates(), underscores are converted to hyphens to
+  // match template file names, and then converted back to hyphens to match
+  // pre-processing and other function names. So if your theme suggestion
+  // contains a hyphen, it will end up as an underscore after this conversion,
+  // and your function names won't be recognized. So, we need to convert
+  // hyphens to underscores in block deltas for the theme suggestions.
+  $variables['theme_hook_suggestions'][] = 'block__' . $variables['block']->module . '__' . strtr($variables['block']->delta, '-', '_');
 
   // Create a valid HTML ID and make sure it is unique.
   $variables['block_html_id'] = drupal_html_id('block-' . $variables['block']->module . '-' . $variables['block']->delta);
diff --git a/modules/block/block.test b/modules/block/block.test
index d036d36..67661be 100644
--- a/modules/block/block.test
+++ b/modules/block/block.test
@@ -666,3 +666,45 @@ class BlockHTMLIdTestCase extends DrupalWebTestCase {
     $this->assertRaw('block-block-test-test-html-id', t('HTML id for test block is valid.'));
   }
 }
+
+
+/**
+ * Unit tests for template_preprocess_block().
+ */
+class BlockTemplateSuggestionsUnitTest extends DrupalUnitTestCase {
+  public static function getInfo() {
+    return array(
+      'name' => 'Block template suggestions',
+      'description' => 'Test the template_preprocess_block() function.',
+      'group' => 'Block',
+    );
+  }
+
+  /**
+   * Test if template_preprocess_block() handles the suggestions right.
+   */
+  function testBlockThemeHookSuggestions() {
+    // Define block delta with underscore to be preprocessed
+    $block1 = new stdClass();
+    $block1->module = 'block';
+    $block1->delta = 'underscore_test';
+    $block1->region = 'footer';
+    $variables1 = array();
+    $variables1['elements']['#block'] = $block1;
+    $variables1['elements']['#children'] = '';
+    template_preprocess_block($variables1);
+    $this->assertEqual($variables1['theme_hook_suggestions'], array('block__footer', 'block__block', 'block__block__underscore_test'), t('Found expected block suggestions for delta with underscore'));
+
+    // Define block delta with hyphens to be preprocessed. Hyphens should be
+    // replaced with underscores.
+    $block2 = new stdClass();
+    $block2->module = 'block';
+    $block2->delta = 'hyphen-test';
+    $block2->region = 'footer';
+    $variables2 = array();
+    $variables2['elements']['#block'] = $block2;
+    $variables2['elements']['#children'] = '';
+    template_preprocess_block($variables2);
+    $this->assertEqual($variables2['theme_hook_suggestions'], array('block__footer', 'block__block', 'block__block__hyphen_test'), t('Hyphens (-) in block delta were replaced by underscore (_)'));
+  }
+}