diff --git a/library/bignum_mod.h b/library/bignum_mod.h
index a3512eb..c4b763f 100644
--- a/library/bignum_mod.h
+++ b/library/bignum_mod.h
@@ -214,14 +214,14 @@
  *                  not be modified in any way until after
  *                  mbedtls_mpi_mod_modulus_free() is called.
  * \param p_limbs   The number of limbs of \p p.
- * \param ored      The optimized reduction structure to use. \p p.
+ * \param modp      A pointer to the optimised reduction function to use. \p p.
  *
  * \return      \c 0 if successful.
  */
 int mbedtls_mpi_mod_optred_modulus_setup(mbedtls_mpi_mod_modulus *N,
                                          const mbedtls_mpi_uint *p,
                                          size_t p_limbs,
-                                         mbedtls_mpi_opt_red_struct *ored);
+                                         int (*modp)(mbedtls_mpi *));
 
 /** Free elements of a modulus structure.
  *