Fixed typo in ActiveRecord.
diff --git a/system/database/DB_active_rec.php b/system/database/DB_active_rec.php
index 2af3553..0238c73 100644
--- a/system/database/DB_active_rec.php
+++ b/system/database/DB_active_rec.php
@@ -1322,7 +1322,7 @@
 		{
 			if ($this->db_debug)
 			{
-				return $this->display_error('db_myst_use_index');
+				return $this->display_error('db_must_use_index');
 			}
 
 			return FALSE;