diff --git a/src/hei_return_code.hpp b/src/hei_return_code.hpp
index ce32468..5d0526e 100644
--- a/src/hei_return_code.hpp
+++ b/src/hei_return_code.hpp
@@ -55,7 +55,7 @@
 
   private:
 
-    const uint32_t iv_rc; ///< return code value
+    uint32_t iv_rc; ///< return code value
 };
 
 /** Function returned successfully. */
@@ -71,5 +71,9 @@
 /** The given chip type has not been initialized. */
 static constexpr ReturnCode RC_CHIP_DATA_MISSING       = ReturnCode(0x00000003);
 
+/** Generic return code indicating something along the hardware register access
+ *  path failed and the returned data is undefined and should not be used. */
+static constexpr ReturnCode RC_REG_ACCESS_FAILURE      = ReturnCode(0x00000004);
+
 } // end namespace libhei