diff --git a/src/register/hei_hardware_register.hpp b/src/register/hei_hardware_register.hpp
index a60b2bd..c6722fb 100755
--- a/src/register/hei_hardware_register.hpp
+++ b/src/register/hei_hardware_register.hpp
@@ -207,7 +207,7 @@
      * @param     RuleChip pointer associated with register
      * @return    Nil
      */
-    void flushCache( ExtensibleChip *i_pChip = NULL ) const;
+    void flushCache( ExtensibleChip *i_pChip = nullptr ) const;
 
   private: // Data