diff --git a/include/mbedtls/platform_util.h b/include/mbedtls/platform_util.h
index b6c50d5..a4fcf31 100644
--- a/include/mbedtls/platform_util.h
+++ b/include/mbedtls/platform_util.h
@@ -225,8 +225,9 @@
  *              cryptographically secure RNG, but provide an RNG for utility
  *              functions.
  *
- * \param num   Max-value for the generated random number.
- *
+ * \param num   Max-value for the generated random number, exclusive.
+ *              The generated number will be on range [0, num).
+ * \return      The generated random number.
  */ 
 uint32_t mbedtls_platform_random_in_range( size_t num );