diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index 44248f3..171d42a 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -715,7 +715,7 @@
  *                 POLARSSL_ERR_MPI_MALLOC_FAILED if memory allocation failed,
  *                 POLARSSL_ERR_MPI_NOT_ACCEPTABLE if X is not prime
  */
-int mpi_is_prime( mpi *X,
+int mpi_is_prime( const mpi *X,
                   int (*f_rng)(void *, unsigned char *, size_t),
                   void *p_rng );