diff --git a/library/bignum_mod_raw.h b/library/bignum_mod_raw.h
index 82a8ce8..60c8d8a 100644
--- a/library/bignum_mod_raw.h
+++ b/library/bignum_mod_raw.h
@@ -53,7 +53,7 @@
  *                 neither its original value nor the value in \p Y.
  */
 void mbedtls_mpi_mod_raw_cond_assign( mbedtls_mpi_uint *X,
-                                      mbedtls_mpi_uint *Y,
+                                      const mbedtls_mpi_uint *Y,
                                       const mbedtls_mpi_mod_modulus *m,
                                       unsigned char assign );