Prevent Warning

Error: Call to a member function getTimestamp() on boolean in islandora_view_datastream_cache_check()

includes/datastream.inc

@@ -161,12 +161,14 @@ function islandora_view_datastream_cache_check(AbstractDatastream $datastream) {
 
   if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])) {
     $modified_since = DateTime::createFromFormat('D, d M Y H:i:s e', $_SERVER['HTTP_IF_MODIFIED_SINCE']);
-    if ($datastream->createdDate->getTimestamp() - $modified_since->getTimestamp() > 0) {
-      // Changed!
-      return $return;
-    }
-    else {
-      $return = 304;
+    if ($modified_since !== FALSE) {
+      if ($datastream->createdDate->getTimestamp() - $modified_since->getTimestamp() > 0) {
+        // Changed!
+        return $return;
+      }
+      else {
+        $return = 304;
+      }
     }
   }
   if ($return === 200 && isset($_SERVER['HTTP_IF_UNMODIFIED_SINCE'])) {