diff --git a/library/bignum_mod.c b/library/bignum_mod.c
index dfccaf4..4382bf8 100644
--- a/library/bignum_mod.c
+++ b/library/bignum_mod.c
@@ -102,7 +102,7 @@
 }
 
 int mbedtls_mpi_mod_modulus_setup( mbedtls_mpi_mod_modulus *m,
-                                   mbedtls_mpi_uint *p,
+                                   const mbedtls_mpi_uint *p,
                                    size_t pn,
                                    mbedtls_mpi_mod_ext_rep ext_rep,
                                    mbedtls_mpi_mod_rep_selector int_rep )