diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
index f651c7f..968cd00 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
@@ -267,9 +267,7 @@ protected function buildLibrariesByExtension($extension) {
       // @todo Convert all uses of #attached[library][]=array('provider','name')
       //   into #attached[library][]='provider/name' and remove this.
       foreach ($library['dependencies'] as $i => $dependency) {
-        if (!is_array($dependency)) {
-          $library['dependencies'][$i] = explode('/', $dependency, 2);
-        }
+        $library['dependencies'][$i] = explode('/', $dependency, 2);
       }
     }
     return $this->libraries[$extension];
diff --git a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryTest.php b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryTest.php
index f8cd82c..fcbb15f 100644
--- a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryTest.php
+++ b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryTest.php
@@ -340,10 +340,9 @@ public function testLibraryWithDependencies() {
     $this->libraryDiscovery->setPaths('module', 'dependencies', $path);
 
     $library = $this->libraryDiscovery->getLibraryByName('dependencies', 'example');
-    $this->assertCount(3, $library['dependencies']);
+    $this->assertCount(2, $library['dependencies']);
     $this->assertEquals(array('external', 'example_external'), $library['dependencies'][0]);
     $this->assertEquals(array('example_module', 'example'), $library['dependencies'][1]);
-    $this->assertEquals(array('example_theme', 'example'), $library['dependencies'][2]);
   }
 
   /**
diff --git a/core/tests/Drupal/Tests/Core/Asset/library_test_files/dependencies.libraries.yml b/core/tests/Drupal/Tests/Core/Asset/library_test_files/dependencies.libraries.yml
index 537e0c3..9a8b4dd 100644
--- a/core/tests/Drupal/Tests/Core/Asset/library_test_files/dependencies.libraries.yml
+++ b/core/tests/Drupal/Tests/Core/Asset/library_test_files/dependencies.libraries.yml
@@ -4,6 +4,3 @@ example:
   dependencies:
     - external/example_external
     - example_module/example
-    -
-      - example_theme
-      - example