diff --git a/library/bignum.c b/library/bignum.c
index 9dc4e4d4f..ae5e7cfa0 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -2090,7 +2090,7 @@ static int mpi_miller_rabin( const mbedtls_mpi *X, size_t rounds,
             j = mbedtls_mpi_bitlen( &A );
             k = mbedtls_mpi_bitlen( &W );
             if (j > k) {
-                MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( &A, j - k ) );
+                A.p[A.n - 1] &= ( (mbedtls_mpi_uint) 1 << ( k - ( A.n - 1 ) * biL - 1 ) ) - 1;
             }
 
             if (count++ > 30) {