diff --git a/include/mbedtls/hmac_drbg.h b/include/mbedtls/hmac_drbg.h
index 3bc675e..2e1108b 100644
--- a/include/mbedtls/hmac_drbg.h
+++ b/include/mbedtls/hmac_drbg.h
@@ -195,11 +195,31 @@
  * \param additional    Additional data to update state with, or NULL
  * \param add_len       Length of additional data, or 0
  *
+ * \return              \c 0 on success, or an error from the underlying
+ *                      hash calculation.
+ *
+ * \note                Additional data is optional, pass NULL and 0 as second
+ *                      third argument if no additional data is being used.
+ */
+int mbedtls_hmac_drbg_update_ret( mbedtls_hmac_drbg_context *ctx,
+                       const unsigned char *additional, size_t add_len );
+
+/**
+ * \brief               HMAC_DRBG update state
+ *
+ * \warning             This function cannot report errors. You should use
+ *                      mbedtls_hmac_drbg_update_ret() instead.
+ *
+ * \param ctx           HMAC_DRBG context
+ * \param additional    Additional data to update state with, or NULL
+ * \param add_len       Length of additional data, or 0
+ *
  * \note                Additional data is optional, pass NULL and 0 as second
  *                      third argument if no additional data is being used.
  */
 void mbedtls_hmac_drbg_update( mbedtls_hmac_drbg_context *ctx,
-                       const unsigned char *additional, size_t add_len );
+                               const unsigned char *additional,
+                               size_t add_len );
 
 /**
  * \brief               HMAC_DRBG reseeding (extracts data from entropy source)