diff --git a/modules/backend/formwidgets/Relation.php b/modules/backend/formwidgets/Relation.php
index 2e2b3b8c4f..e15462fcd7 100644
--- a/modules/backend/formwidgets/Relation.php
+++ b/modules/backend/formwidgets/Relation.php
@@ -3,7 +3,9 @@
 use Db;
 use Backend\Classes\FormField;
 use Backend\Classes\FormWidgetBase;
+use Lang;
 use Winter\Storm\Database\Relations\Relation as RelationBase;
+use Winter\Storm\Exception\SystemException;
 
 /**
  * Form Relationship
@@ -95,6 +97,7 @@ public function prepareVars()
 
     /**
      * Makes the form object used for rendering a simple field type
+     * @throws SystemException if an unsupported relation type is used.
      */
     protected function makeRenderFormField()
     {
@@ -110,9 +113,14 @@ protected function makeRenderFormField()
 
             if (in_array($relationType, ['belongsToMany', 'morphToMany', 'morphedByMany', 'hasMany'])) {
                 $field->type = 'checkboxlist';
-            }
-            elseif (in_array($relationType, ['belongsTo', 'hasOne'])) {
+            } elseif (in_array($relationType, ['belongsTo', 'hasOne'])) {
                 $field->type = 'dropdown';
+            } else {
+                throw new SystemException(
+                    Lang::get('backend::lang.relation.relationwidget_unsupported_type', [
+                        'type' => $relationType
+                    ])
+                );
             }
 
             // Order query by the configured option.
diff --git a/modules/backend/lang/en/lang.php b/modules/backend/lang/en/lang.php
index 6fbafc7a96..6fe8b20f31 100644
--- a/modules/backend/lang/en/lang.php
+++ b/modules/backend/lang/en/lang.php
@@ -320,6 +320,7 @@
         'missing_model' => 'Relation behavior used in :class does not have a model defined.',
         'invalid_action_single' => 'This action cannot be performed on a singular relationship.',
         'invalid_action_multi' => 'This action cannot be performed on a multiple relationship.',
+        'relationwidget_unsupported_type' => 'The ":type" relation type is unsupported by the Relation widget.',
         'help' => 'Click on an item to add',
         'related_data' => 'Related :name data',
         'add' => 'Add',
diff --git a/modules/backend/lang/fr/lang.php b/modules/backend/lang/fr/lang.php
index 13a5ee534a..bc3ded4eac 100644
--- a/modules/backend/lang/fr/lang.php
+++ b/modules/backend/lang/fr/lang.php
@@ -297,6 +297,7 @@
         'missing_model' => 'La behavior relation utilisée dans la classe :class n’a pas de modèle défini.',
         'invalid_action_single' => 'Cette action ne peut être effectuée sur une relation unitaire.',
         'invalid_action_multi' => 'Cette action ne peut être effectuée sur une relation multiple.',
+        'relationwidget_unsupported_type' => 'Le type de relation ":type" n\'est pas supporté par le widget Relation.',
         'help' => 'Cliquer sur un élément pour l’ajouter',
         'related_data' => 'Donnée :name liée',
         'add' => 'Ajouter',