diff --git a/group.module b/group.module
index 2d156d1..8c45d9a 100644
--- a/group.module
+++ b/group.module
@@ -204,12 +204,10 @@ function group_form_block_form_alter(&$form, FormStateInterface $form_state, $fo
  * Implements hook_entity_delete().
  */
 function group_entity_delete(EntityInterface $entity) {
-  if ($entity instanceof ContentEntityInterface) {
-    if ($group_contents = GroupContent::loadByEntity($entity)) {
-      /** @var \Drupal\group\Entity\GroupContent $group_content */
-      foreach ($group_contents as $group_content) {
-        $group_content->delete();
-      }
+  if ($group_contents = GroupContent::loadByEntity($entity)) {
+    /** @var \Drupal\group\Entity\GroupContent $group_content */
+    foreach ($group_contents as $group_content) {
+      $group_content->delete();
     }
   }
 }
@@ -336,7 +334,7 @@ function group_form_alter(&$form, FormStateInterface $form_state, $form_id) {
   // GroupContentController::createForm() tends to load entity forms for adding
   // entities to a group. We need to add or alter the submit handlers of those
   // forms for the process to work properly.
-  if ($form_state->getFormObject() instanceof ContentEntityForm && $form_state->has('group_content_enabler')) {
+  if ($form_state->getFormObject() && $form_state->has('group_content_enabler')) {
     if ($wizard = $form_state->has('store_id')) {
       $store = \Drupal::service('user.private_tempstore')->get('group_content_wizard');
       $store_id = $form_state->get('store_id');
diff --git a/src/Entity/Group.php b/src/Entity/Group.php
index 9066920..79b7811 100644
--- a/src/Entity/Group.php
+++ b/src/Entity/Group.php
@@ -2,12 +2,12 @@
 
 namespace Drupal\group\Entity;
 
-use Drupal\Core\Field\BaseFieldDefinition;
 use Drupal\Core\Entity\ContentEntityBase;
-use Drupal\Core\Entity\ContentEntityInterface;
-use Drupal\Core\Entity\EntityTypeInterface;
 use Drupal\Core\Entity\EntityChangedTrait;
+use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\Core\Entity\EntityTypeInterface;
+use Drupal\Core\Field\BaseFieldDefinition;
 use Drupal\Core\Session\AccountInterface;
 use Drupal\user\UserInterface;
 
@@ -149,7 +149,7 @@ class Group extends ContentEntityBase implements GroupInterface {
   /**
    * {@inheritdoc}
    */
-  public function addContent(ContentEntityInterface $entity, $plugin_id, $values = []) {
+  public function addContent(EntityInterface $entity, $plugin_id, $values = []) {
     $plugin = $this->getGroupType()->getContentPlugin($plugin_id);
 
     // Only add the entity if the provided plugin supports it.
diff --git a/src/Entity/GroupContent.php b/src/Entity/GroupContent.php
index 3872844..a364a98 100644
--- a/src/Entity/GroupContent.php
+++ b/src/Entity/GroupContent.php
@@ -2,13 +2,13 @@
 
 namespace Drupal\group\Entity;
 
-use Drupal\Core\Entity\ContentEntityInterface;
-use Drupal\user\UserInterface;
-use Drupal\Core\Field\BaseFieldDefinition;
 use Drupal\Core\Entity\ContentEntityBase;
-use Drupal\Core\Entity\EntityTypeInterface;
 use Drupal\Core\Entity\EntityChangedTrait;
+use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\Core\Entity\EntityTypeInterface;
+use Drupal\Core\Field\BaseFieldDefinition;
+use Drupal\user\UserInterface;
 
 /**
  * Defines the Group content entity.
@@ -120,7 +120,7 @@ class GroupContent extends ContentEntityBase implements GroupContentInterface {
   /**
    * {@inheritdoc}
    */
-  public static function loadByEntity(ContentEntityInterface $entity) {
+  public static function loadByEntity(EntityInterface $entity) {
     $group_content_types = GroupContentType::loadByEntityTypeId($entity->getEntityTypeId());
 
     // If no responsible group content types were found, we return nothing.
@@ -256,9 +256,13 @@ class GroupContent extends ContentEntityBase implements GroupContentInterface {
 
     // Borrowed this logic from the Comment module.
     // Warning! May change in the future: https://www.drupal.org/node/2346347
+    // The target type is set to a config entity to force a string field
+    // for the entity ID.
+    // @see https://www.drupal.org/node/1757452.
     $fields['entity_id'] = BaseFieldDefinition::create('entity_reference')
       ->setLabel(t('Content'))
       ->setDescription(t('The entity to add to the group.'))
+      ->setSetting('target_type', 'group_type')
       ->setDisplayOptions('form', [
         'type' => 'entity_reference_autocomplete',
         'weight' => 5,
diff --git a/src/Entity/GroupContentInterface.php b/src/Entity/GroupContentInterface.php
index 28965aa..dedcf03 100644
--- a/src/Entity/GroupContentInterface.php
+++ b/src/Entity/GroupContentInterface.php
@@ -2,9 +2,10 @@
 
 namespace Drupal\group\Entity;
 
-use Drupal\user\EntityOwnerInterface;
 use Drupal\Core\Entity\ContentEntityInterface;
 use Drupal\Core\Entity\EntityChangedInterface;
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\user\EntityOwnerInterface;
 
 /**
  * Provides an interface defining a Group content entity.
@@ -55,12 +56,12 @@ interface GroupContentInterface extends ContentEntityInterface, EntityOwnerInter
   /**
    * Loads group content entities which reference a given entity.
    *
-   * @param \Drupal\Core\Entity\ContentEntityInterface $entity
+   * @param \Drupal\Core\Entity\EntityInterface $entity
    *   An entity which may be within one or more groups.
    *
    * @return \Drupal\group\Entity\GroupContentInterface[]
    *   An array of group content entities which reference the given entity.
    */
-  public static function loadByEntity(ContentEntityInterface $entity);
+  public static function loadByEntity(EntityInterface $entity);
 
 }
diff --git a/src/Entity/GroupInterface.php b/src/Entity/GroupInterface.php
index f6477fc..ef65ef8 100644
--- a/src/Entity/GroupInterface.php
+++ b/src/Entity/GroupInterface.php
@@ -2,10 +2,11 @@
 
 namespace Drupal\group\Entity;
 
-use Drupal\user\EntityOwnerInterface;
 use Drupal\Core\Entity\ContentEntityInterface;
 use Drupal\Core\Entity\EntityChangedInterface;
+use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Session\AccountInterface;
+use Drupal\user\EntityOwnerInterface;
 use Drupal\user\UserInterface;
 
 /**
@@ -33,7 +34,7 @@ interface GroupInterface extends ContentEntityInterface, EntityOwnerInterface, E
   /**
    * Adds a content entity as a group content entity.
    *
-   * @param \Drupal\Core\Entity\ContentEntityInterface $entity
+   * @param \Drupal\Core\Entity\EntityInterface $entity
    *   The content entity to add to the group.
    * @param string $plugin_id
    *   The ID of the content enabler plugin to add the entity with.
@@ -41,7 +42,7 @@ interface GroupInterface extends ContentEntityInterface, EntityOwnerInterface, E
    *   (optional) Extra values to add to the group content relationship. You
    *   cannot overwrite the group ID (gid) or entity ID (entity_id).
    */
-  public function addContent(ContentEntityInterface $entity, $plugin_id, $values = []);
+  public function addContent(EntityInterface $entity, $plugin_id, $values = []);
 
   /**
    * Retrieves all GroupContent entities for the group.