diff -u b/core/modules/rdf/rdf.module b/core/modules/rdf/rdf.module
--- b/core/modules/rdf/rdf.module
+++ b/core/modules/rdf/rdf.module
@@ -567,7 +567,8 @@
  */
 function template_preprocess_rdf_metadata(&$variables) {
   foreach ($variables['metadata'] as $key => $attributes) {
-    // In some cases $attributes is unset, so we set it to an empty array.
+    // In some cases $attributes is not set. In those cases make it an array
+    // for when we construct the Attribute object below.
     if (!isset($attributes)) {
       $attributes = array();
     }