diff --git a/libyara/include/yara/types.h b/libyara/include/yara/types.h
index 973aba9f58..36aac97cf4 100644
--- a/libyara/include/yara/types.h
+++ b/libyara/include/yara/types.h
@@ -293,8 +293,13 @@ struct YR_RULE
   // Number of atoms generated for this rule.
   int32_t num_atoms;
 
+  // Number of strings that must match for this rule to have some possibility
+  // to match.
   uint32_t required_strings;
 
+  // Just for padding.
+  uint32_t unused;
+
   DECLARE_REFERENCE(const char*, identifier);
   DECLARE_REFERENCE(const char*, tags);
   DECLARE_REFERENCE(YR_META*, metas);