diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index aa51556..a713313 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -665,8 +665,8 @@
  *
  * \return         0 if successful,
  *                 MBEDTLS_ERR_MPI_ALLOC_FAILED if memory allocation failed,
- *                 MBEDTLS_ERR_MPI_BAD_INPUT_DATA if N is negative or nil
-                   MBEDTLS_ERR_MPI_NOT_ACCEPTABLE if A has no inverse mod N
+ *                 MBEDTLS_ERR_MPI_BAD_INPUT_DATA if N is <= 1,
+                   MBEDTLS_ERR_MPI_NOT_ACCEPTABLE if A has no inverse mod N.
  */
 int mbedtls_mpi_inv_mod( mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi *N );