diff --git a/library/common.h b/library/common.h
index 5845766..f4cef97 100644
--- a/library/common.h
+++ b/library/common.h
@@ -29,6 +29,7 @@
 #include "mbedtls/config.h"
 #endif
 
+#if defined(MBEDTLS_TEST_HOOKS)
 /** Helper to define a function as static except when building invasive tests.
  *
  * If a function is only used inside its own source file and should be
@@ -44,10 +45,25 @@
  * #endif
  * ```
  */
-#if defined(MBEDTLS_TEST_HOOKS)
 #define MBEDTLS_STATIC_TESTABLE
+
+/** Helper macro and function to combine a high and low level error code.
+ * 
+ * This function uses a hook (`mbedtls_test_err_add_hook`) to allow invasive
+ * testing of its inputs. This is used in the test infrastructure to report
+ * on errors when combining two error codes of the same level (e.g: two high
+ * or two low level errors).
+ */
+int mbedtls_err_add( int high, int low, const char *file, int line );
+#define MBEDTLS_ERR_ADD( high, low )  \
+    ( mbedtls_err_add( high, low, __FILE__, __LINE__ ) )
+
 #else
 #define MBEDTLS_STATIC_TESTABLE static
-#endif
+
+#define MBEDTLS_ERR_ADD( high, low ) \
+    ( high + low )
+
+#endif /* MBEDTLS_TEST_HOOKS */
 
 #endif /* MBEDTLS_LIBRARY_COMMON_H */