diff --git a/modules/cms/models/ThemeData.php b/modules/cms/models/ThemeData.php
index 941305405..c03af4dc5 100644
--- a/modules/cms/models/ThemeData.php
+++ b/modules/cms/models/ThemeData.php
@@ -74,6 +74,15 @@ class ThemeData extends Model
 
     public function afterFetch()
     {
+        /*
+         * Repeater form fields store arrays and must be jsonable.
+         */
+        foreach ($this->getFormFields() as $id => $field) {
+            if (isset($field['type']) && $field['type'] == 'repeater') {
+                $this->jsonable[] = $id;
+            }
+        }
+
         /*
          * Fill this model with the jsonable attributes kept in 'data'.
          */