diff --git a/dfp.module b/dfp.module
index 20984c5..31b98bb 100755
--- a/dfp.module
+++ b/dfp.module
@@ -898,6 +898,18 @@ function dfp_token_replace($text, $tag = NULL, array $options = array()) {
 }
 
 /**
+ * Calls the Entity Modified module's entity_modified_last() function safely.
+ */
+function _dfp_entity_modified_last($entity_type, $entity) {
+  if (module_load('module', 'entity_modified') && function_exists('entity_modified_last')) {
+    return entity_modified_last($entity_type, $entity);
+  }
+  static $key = 1000;
+  watchdog(WATCHDOG_ERROR, 'DFP module failed to call the entity_modified_last function.');
+  return (string)$key++;
+}
+
+/**
  * Generates an identifying key for the lookup to be processed.
  *
  * @param string $text
@@ -915,10 +927,10 @@ function _dfp_token_replace_make_key($text, array $data) {
   // be used as the cache key itself we're not limited to 255 characters but
   // it will be nicer on array lookups in PHP.
   $keys[] = sha1($text);
-  $keys[] = isset($data['node']->nid) ? $data['node']->nid . '-' . entity_modified_last('node', $data['node'])  : NULL;
-  $keys[] = isset($data['user']->uid) ? $data['user']->uid . '-' . entity_modified_last('user', $data['user']) : NULL;
-  $keys[] = isset($data['term']->tid) ? $data['term']->tid . '-' . entity_modified_last('taxonomy_term', $data['term']) : NULL;
-  $keys[] = isset($data['tag']->machinename) ? $data['tag']->machinename . '-' . entity_modified_last('tag', $data['tag']) : NULL;
+  $keys[] = isset($data['node']->nid) ? $data['node']->nid . '-' . _dfp_entity_modified_last('node', $data['node'])  : NULL;
+  $keys[] = isset($data['user']->uid) ? $data['user']->uid . '-' . _dfp_entity_modified_last('user', $data['user']) : NULL;
+  $keys[] = isset($data['term']->tid) ? $data['term']->tid . '-' . _dfp_entity_modified_last('taxonomy_term', $data['term']) : NULL;
+  $keys[] = isset($data['tag']->machinename) ? $data['tag']->machinename . '-' . _dfp_entity_modified_last('tag', $data['tag']) : NULL;
 
   return implode('|', array_filter($keys));
 }