diff --git a/h5p.classes.php b/h5p.classes.php
index e4b3d20..0e2edc1 100644
--- a/h5p.classes.php
+++ b/h5p.classes.php
@@ -1375,6 +1375,14 @@ class H5PContentValidator {
         }
       }
     }
+    foreach ($semantics->fields as $field) {
+      if (!(isset($field->optional) && $field->optional)) {
+        // Check if field is in group.
+        if (! property_exists($group, $field->name)) {
+          $this->h5pF->setErrorMessage($this->h5pF->t('No value given for mandatory field ' . $field->name));
+        }
+      }
+    }
   }
 
   /**