diff --git a/include/mbedtls/ctr_drbg.h b/include/mbedtls/ctr_drbg.h
index 17a5cfc4e..fbad4a760 100644
--- a/include/mbedtls/ctr_drbg.h
+++ b/include/mbedtls/ctr_drbg.h
@@ -223,7 +223,8 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx );
  *                      string is empty regardless of the value of \p len.
  * \param len           The length of the personalization string.
  *                      This must be at most
- *                      #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT / 2.
+ *                      #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT
+ *                      - #MBEDTLS_CTR_DRBG_ENTROPY_LEN.
  *
  * \return              \c 0 on success, or
  *                      #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED on failure.