diff --git a/include/mbedtls/platform_util.h b/include/mbedtls/platform_util.h
index 8d00eba..1f45b1c 100644
--- a/include/mbedtls/platform_util.h
+++ b/include/mbedtls/platform_util.h
@@ -161,8 +161,11 @@
  * \param buf   Buffer to be zeroized
  * \param len   Length of the buffer in bytes
  *
+ * \return      The value of \p buf if the operation was successful.
+ * \return      NULL if a potential FI attack was detected or input parameters
+ *              are not valid.
  */
-void mbedtls_platform_zeroize( void *buf, size_t len );
+void *mbedtls_platform_zeroize( void *buf, size_t len );
 
 /**
  * \brief       Secure memset
@@ -176,7 +179,8 @@
  * \param value Value to be used when setting the buffer.
  * \param num   The length of the buffer in bytes.
  *
- * \return      The value of \p ptr.
+ * \return      The value of \p ptr if the operation was successful.
+ * \return      NULL if a potential FI attack was detected.
  */
 void *mbedtls_platform_memset( void *ptr, int value, size_t num );