diff --git a/library/bignum_core.h b/library/bignum_core.h
index 64d51b9..e5500f1 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -178,7 +178,9 @@
  * \param[in]  A        The address of the source MPI. This must be initialized.
  * \param      limbs    The number of limbs of \p A.
  * \param      assign   The condition deciding whether to perform the
- *                      assignment or not.
+ *                      assignment or not. Callers will need to use
+ *                      the constant time interface (e.g. `mbedtls_ct_bool()`)
+ *                      to construct this argument.
  *
  * \note           This function avoids leaking any information about whether
  *                 the assignment was done or not.