diff --git a/core/lib/Drupal/Core/Block/BlockBase.php b/core/lib/Drupal/Core/Block/BlockBase.php
index 12b93ea..d06960d 100644
--- a/core/lib/Drupal/Core/Block/BlockBase.php
+++ b/core/lib/Drupal/Core/Block/BlockBase.php
@@ -277,10 +277,11 @@ public function setTransliteration(TransliterationInterface $transliteration) {
    * {@inheritdoc}
    */
   public function getFormClass($operation) {
-    if ($this->hasFormClass($operation)) {
-      $plugin_definition = $this->getPluginDefinition();
-      // Fallback to the class of this plugin if the key is not set.
-      return isset($plugin_definition['forms'][$operation]) ? $plugin_definition['forms'][$operation] : get_class($this);
+    if (isset($this->getPluginDefinition()['forms'][$operation])) {
+      return $this->getPluginDefinition()['forms'][$operation];
+    }
+    elseif ($operation == 'configure' && $this instanceof PluginFormInterface) {
+      return static::CLASS;
     }
   }
 
@@ -288,7 +289,7 @@ public function getFormClass($operation) {
    * {@inheritdoc}
    */
   public function hasFormClass($operation) {
-    return isset($this->getPluginDefinition()['forms'][$operation]) || ($operation == 'configure' && $this instanceof PluginFormInterface);
+    return !empty($this->getFormClass($operation));
   }
 
 }
diff --git a/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php b/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php
index feac004..c0423bc 100644
--- a/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php
@@ -383,7 +383,6 @@ public function providerTestProcessDefinition() {
     $data['no_form'][] = ['class' => TestPluginForm::class];
     $data['no_form'][] = [
       'class' => TestPluginForm::class,
-      'forms' => ['configure' => TestPluginForm::class],
       'foo' => ['bar' => ['baz']],
     ];