diff --git a/library/bignum_core.h b/library/bignum_core.h
index 493ec12cdc..bd246d5e36 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -79,7 +79,7 @@ void mbedtls_mpi_core_bigendian_to_host( mbedtls_mpi_uint *A,
  *          the condition was true or not.
  *
  * \param[out] X        The address of the first MPI. This must be initialized.
- *                      It must have at least \p limbs limbs.
+ *                      Must have enough limbs to store the full value of \p A.
  * \param[in]  A        The address of the second MPI. This must be initialized.
  * \param      limbs    The number of limbs of \p A.
  * \param      assign   The condition deciding whether to perform the
diff --git a/library/bignum_mod_raw.h b/library/bignum_mod_raw.h
index 231bb72d38..845ebaa5a7 100644
--- a/library/bignum_mod_raw.h
+++ b/library/bignum_mod_raw.h
@@ -37,7 +37,10 @@
  * \brief   Perform a safe conditional copy of MPI which doesn't reveal whether
  *          the condition was true or not.
  *
+ * The size to copy is determined by \p N.
+ *
  * \param[out] X        The address of the first MPI. This must be initialized.
+ *                      Must have enough limbs to store the full value of \p A.
  * \param[in]  A        The address of the second MPI. This must be initialized.
  * \param[in]  N        The address of the modulus related to \p X and \p A.
  * \param      assign   The condition deciding whether to perform the
@@ -61,6 +64,8 @@ void mbedtls_mpi_mod_raw_cond_assign( mbedtls_mpi_uint *X,
  * \brief   Perform a safe conditional swap of MPI which doesn't reveal whether
  *          the condition was true or not.
  *
+ * The size to swap is determined by \p N.
+ *
  * \param[in,out] X     The address of the first MPI. This must be initialized.
  * \param[in,out] Y     The address of the second MPI. This must be initialized.
  * \param[in]     N     The address of the modulus related to \p X and \p Y.