diff --git a/library/bignum_mod.c b/library/bignum_mod.c
index 70bb584..54d38bd 100644
--- a/library/bignum_mod.c
+++ b/library/bignum_mod.c
@@ -88,7 +88,7 @@
             N->rep.mont.mm = 0;
             break;
         case MBEDTLS_MPI_MOD_REP_OPT_RED:
-            mbedtls_free(N->rep.ored);
+            N->rep.ored.modp = NULL;
             break;
         case MBEDTLS_MPI_MOD_REP_INVALID:
             break;
@@ -166,10 +166,10 @@
 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 *))
 {
     standard_modulus_setup(N, p, p_limbs, MBEDTLS_MPI_MOD_REP_OPT_RED);
-    N->rep.ored = ored;
+    N->rep.ored.modp = modp;
     return 0;
 }
 
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.
  *