diff --git a/library/bignum_mod_raw.h b/library/bignum_mod_raw.h
index f9968ba..73eaf18 100644
--- a/library/bignum_mod_raw.h
+++ b/library/bignum_mod_raw.h
@@ -178,6 +178,10 @@
  * \brief          Returns the number of limbs of working memory required for
  *                 a call to `mbedtls_mpi_mod_raw_inv_prime()`.
  *
+ * \note           This will always be at least
+ *                 `mbedtls_mpi_core_montmul_working_limbs(AN_limbs)`,
+ *                 i.e. sufficient for a call to `mbedtls_mpi_core_montmul()`.
+ *
  * \param AN_limbs The number of limbs in the input `A` and the modulus `N`
  *                 (they must be the same size) that will be given to
  *                 `mbedtls_mpi_mod_raw_inv_prime()`.