diff --git a/include/mbedtls/platform_util.h b/include/mbedtls/platform_util.h
index 6f09209..8d00eba 100644
--- a/include/mbedtls/platform_util.h
+++ b/include/mbedtls/platform_util.h
@@ -262,6 +262,10 @@
  *
  *              Duration of the delay is random as number of variable increments
  *              is randomized.
+ *
+ * \note        This function works only if the MBEDTLS_FI_COUNTERMEASURES flag
+ *              is defined in the configuration. Otherwise, the function does
+ *              nothing.
  */
 void mbedtls_platform_random_delay( void );