diff --git a/core/lib/Drupal/Core/Plugin/ContextAwarePluginAssignmentTrait.php b/core/lib/Drupal/Core/Plugin/ContextAwarePluginAssignmentTrait.php
index 299c209..746ad3d 100644
--- a/core/lib/Drupal/Core/Plugin/ContextAwarePluginAssignmentTrait.php
+++ b/core/lib/Drupal/Core/Plugin/ContextAwarePluginAssignmentTrait.php
@@ -67,6 +67,7 @@ protected function addContextAssignmentElement(ContextAwarePluginInterface $plug
           $element[$context_slot]['#empty_value'] = '';
         }
         elseif (count($options) == 1) {
+          $element[$context_slot]['#default_value'] = key($options);
           $element[$context_slot]['#disabled'] = TRUE;
           $element[$context_slot]['#description'] .= '<p>' . $this->t('<strong>Note:</strong> This field cannot be changed because there is only one context item available and it is required.') . '</p>';
         }
diff --git a/core/modules/block/src/Tests/BlockLanguageTest.php b/core/modules/block/src/Tests/BlockLanguageTest.php
index 265524f..13d7cf2 100644
--- a/core/modules/block/src/Tests/BlockLanguageTest.php
+++ b/core/modules/block/src/Tests/BlockLanguageTest.php
@@ -54,7 +54,8 @@ public function testLanguageBlockVisibility() {
     $this->drupalGet('admin/structure/block/add/system_powered_by_block' . '/' . $default_theme);
 
     $this->assertField('visibility[language][langcodes][en]', 'Language visibility field is visible.');
-    $this->assertNoField('visibility[language][context_mapping][language]', 'Language type field is not visible.');
+    $this->assertField('visibility[language][context_mapping][language]', 'Language type field is visible.');
+    $this->assertEqual((string) $this->xpath('//select[@name="visibility[language][context_mapping][language]"]/@disabled')[0], 'disabled');
 
     // Enable a standard block and set the visibility setting for one language.
     $edit = array(