diff --git a/library/bignum.c b/library/bignum.c
index 47a7b98e8..0c758c509 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -2257,7 +2257,7 @@ static int mpi_select( mbedtls_mpi *R, const mbedtls_mpi *T, size_t T_size, size
     for( size_t i = 0; i < T_size; i++ )
     {
         MBEDTLS_MPI_CHK( mbedtls_mpi_safe_cond_assign( R, &T[i],
-                                        mbedtls_mpi_cf_bool_eq( i, idx ) ) );
+                        (unsigned char) mbedtls_mpi_cf_bool_eq( i, idx ) ) );
     }
 
 cleanup:
@@ -2441,7 +2441,7 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
             /*
              * X = X * W[wbits] R^-1 mod N
              */
-            MBEDTLS_MPI_CHK( mpi_select( &WW, W, 1 << wsize, wbits ) );
+            MBEDTLS_MPI_CHK( mpi_select( &WW, W, (size_t) 1 << wsize, wbits ) );
             mpi_montmul( X, &WW, N, mm, &T );
 
             state--;