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
@@ -567,6 +567,9 @@
  */
 function template_preprocess_rdf_metadata(&$variables) {
   foreach ($variables['metadata'] as $key => $attributes) {
+    if (!isset($attributes)) {
+      $attributes = array();
+    }
     // The XHTML+RDFa doctype allows either <span></span> or <span /> syntax to
     // be used, but for maximum browser compatibility, W3C recommends the
     // former when serving pages using the text/html media type, see