diff --git a/core/lib/Drupal/Core/Plugin/ContextAwarePluginAssignmentTrait.php b/core/lib/Drupal/Core/Plugin/ContextAwarePluginAssignmentTrait.php
index f9e349ed69e0a035b55616b4dd2ec9b8645da090..012bd30e7303d893528fe60c8cd1f1590b3bff33 100644
--- a/core/lib/Drupal/Core/Plugin/ContextAwarePluginAssignmentTrait.php
+++ b/core/lib/Drupal/Core/Plugin/ContextAwarePluginAssignmentTrait.php
@@ -53,7 +53,7 @@ protected function addContextAssignmentElement(ContextAwarePluginInterface $plug
         ];
       }
 
-      if (count($options) > 1) {
+      if (count($options) > 1 || !$definition->isRequired()) {
         $assignments = $plugin->getContextMapping();
         $element[$context_slot] = [
           '#title' => $definition->getLabel() ?: $this->t('Select a @context value:', ['@context' => $context_slot]),
@@ -63,6 +63,9 @@ protected function addContextAssignmentElement(ContextAwarePluginInterface $plug
           '#default_value' => !empty($assignments[$context_slot]) ? $assignments[$context_slot] : '',
           '#description' => $definition->getDescription(),
         ];
+        if (!$definition->isRequired()) {
+          $element[$context_slot]['#empty_value'] = '';
+        }
       }
     }
     return $element;
diff --git a/core/lib/Drupal/Core/Plugin/ContextAwarePluginBase.php b/core/lib/Drupal/Core/Plugin/ContextAwarePluginBase.php
index 21ee311c0e298c80399d0cc80cf2dc72eebbe431..219dd37e23e5ec081868439a38849d11306b3418 100644
--- a/core/lib/Drupal/Core/Plugin/ContextAwarePluginBase.php
+++ b/core/lib/Drupal/Core/Plugin/ContextAwarePluginBase.php
@@ -93,7 +93,7 @@ public function getContextMapping() {
   public function setContextMapping(array $context_mapping) {
     if ($this instanceof ConfigurablePluginInterface) {
       $configuration = $this->getConfiguration();
-      $configuration['context_mapping'] = $context_mapping;
+      $configuration['context_mapping'] = array_filter($context_mapping);
       $this->setConfiguration($configuration);
     }
     else {
diff --git a/core/modules/block/src/Tests/BlockUiTest.php b/core/modules/block/src/Tests/BlockUiTest.php
index 532094265726dddffc5edc2e2046509fc79f14ee..25b7942a59e12c33a7ef0f39877f16b6f567ab99 100644
--- a/core/modules/block/src/Tests/BlockUiTest.php
+++ b/core/modules/block/src/Tests/BlockUiTest.php
@@ -207,7 +207,6 @@ public function testContextAwareBlocks() {
     $this->assertTrue(!empty($elements), 'The context-aware test block appears.');
     $definition = \Drupal::service('plugin.manager.block')->getDefinition('test_context_aware');
     $this->assertTrue(!empty($definition), 'The context-aware test block exists.');
-
     $edit = [
       'region' => 'content',
       'settings[context_mapping][user]' => '@block_test.multiple_static_context:user2',
@@ -217,6 +216,15 @@ public function testContextAwareBlocks() {
     $this->drupalGet('');
     $this->assertText('Test context-aware block');
     $this->assertRaw($expected_text);
+
+    // Test context mapping allows empty selection for optional contexts.
+    $this->drupalGet('admin/structure/block/manage/testcontextawareblock');
+    $edit = [
+      'settings[context_mapping][user]' => '',
+    ];
+    $this->drupalPostForm(NULL, $edit, 'Save block');
+    $this->drupalGet('');
+    $this->assertText('No context mapping selected.');
   }
 
   /**
diff --git a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareBlock.php b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareBlock.php
index c4223d0ea79f94b7e52cf37c28596ea4cfcb66fd..013dbae75b8a7e3f1861ed084b8353c5c8d5414e 100644
--- a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareBlock.php
+++ b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareBlock.php
@@ -16,7 +16,7 @@
  *   id = "test_context_aware",
  *   admin_label = @Translation("Test context-aware block"),
  *   context = {
- *     "user" = @ContextDefinition("entity:user")
+ *     "user" = @ContextDefinition("entity:user", required = FALSE)
  *   }
  * )
  */
@@ -31,7 +31,7 @@ public function build() {
     return array(
       '#prefix' => '<div id="' . $this->getPluginId() . '--username">',
       '#suffix' => '</div>',
-      '#markup' => $user->getUsername(),
+      '#markup' => $user ? $user->getUsername() : 'No context mapping selected.' ,
     );
   }