diff --git a/include/mbedtls/error.h b/include/mbedtls/error.h
index 39874b9c4..624228914 100644
--- a/include/mbedtls/error.h
+++ b/include/mbedtls/error.h
@@ -133,10 +133,14 @@ extern void (*mbedtls_test_hook_error_add)( int, int, const char *, int );
  * \brief Combines a high-level and low-level error code together.
  *
  *        This function can be called directly however it is usually
- *        called via the mbedtls_error_add macro.
+ *        called via the #MBEDTLS_ERROR_ADD macro.
  *
- * \note  When invasive testing is enabled via MBEDTLS_TEST_HOOKS also try to
- *        call mbedtls_test_hook_error_add.
+ *        While a value of zero is not a negative error code, it is still an
+ *        error code (that denotes success) and can be combined with both a
+ *        negative error code or another value of zero.
+ *
+ * \note  When invasive testing is enabled via #MBEDTLS_TEST_HOOKS, also try to
+ *        call \link mbedtls_test_hook_error_add \endlink.
  *
  * \param high      high-level error code. See error.h for more details.
  * \param low       low-level error code. See error.h for more details.
diff --git a/tests/src/helpers.c b/tests/src/helpers.c
index 9c1198ea3..b54661195 100644
--- a/tests/src/helpers.c
+++ b/tests/src/helpers.c
@@ -292,7 +292,7 @@ void mbedtls_test_err_add_check( int high, int low,
      * examples given in comments have been made positive for ease of
      * understanding. The structure of an error code is such:
      *
-     *                                                shhhhhhhllllllll
+     *                                                shhhhhhhhlllllll
      *
      * s = sign bit.
      * h = high level error code (includes high and module error codes).
@@ -308,7 +308,7 @@ void mbedtls_test_err_add_check( int high, int low,
         mbedtls_test_fail( "'high' error code is greater than 15 bits",
                             line, file );
     }
-    else if ( ( high & 0x7F ) != 0 )    // high & 0000000011111111
+    else if ( ( high & 0x7F ) != 0 )    // high & 0000000001111111
     {
         mbedtls_test_fail( "'high' contains a low-level error code",
                             line, file );