diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index ed0c4e798e..c67d653d7a 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -993,8 +993,9 @@ int mbedtls_mpi_gcd(mbedtls_mpi *G, const mbedtls_mpi *A,
  *                 to an initialized MPI. This value can be negative, in which
  *                 case a positive answer will still be returned in \p X.
  * \param N        The base of the modular inversion. This must point to an
- *                 initialized MPI. If this points to the same MPI as \p X,
- *                 then the value returned in \p X will be incorrect.
+ *                 initialized MPI and be greater than one. If this points to
+ *                 the same MPI as \p X, then the value returned in \p X will
+ *                 be incorrect.
  *
  * \return         \c 0 if successful.
  * \return         #MBEDTLS_ERR_MPI_ALLOC_FAILED if a memory allocation failed.