diff --git a/grn.drush.inc b/grn.drush.inc
index 787c12b..068cb0f 100644
--- a/grn.drush.inc
+++ b/grn.drush.inc
@@ -62,6 +62,15 @@ function grn_drush_command() {
 }
 
 /**
+ * Implementation of drush_hook_COMMAND_validate().
+ */
+function drush_grn_release_notes_validate($tag1n = NULL, $tag2n = NULL) {
+  if (is_null($tag1n) || is_null($tag2n)) {
+    drush_set_error('DRUSH_GRN_MISSING_ARG', dt('Two valid Git tags are required as arguments.'));
+  }
+}
+
+/**
  * Implementation of drush_hook_COMMAND().
  */
 function drush_grn_release_notes($tag1n, $tag2n) {