diff --git a/library/pkwrite.c b/library/pkwrite.c
index 150626c14..f9e598c93 100644
--- a/library/pkwrite.c
+++ b/library/pkwrite.c
@@ -472,8 +472,8 @@ int mbedtls_pk_write_key_der( mbedtls_pk_context *key, unsigned char *buf, size_
  *      otherPrimeInfos   OtherPrimeInfos OPTIONAL  0 (not supported)
  *  }
  */
-#define MPI_MAX_SIZE_2          MBEDTLS_MPI_MAX_SIZE / 2 + \
-                                MBEDTLS_MPI_MAX_SIZE % 2
+#define MPI_MAX_SIZE_2          ( MBEDTLS_MPI_MAX_SIZE / 2 + \
+                                  MBEDTLS_MPI_MAX_SIZE % 2 )
 #define RSA_PRV_DER_MAX_BYTES   47 + 3 * MBEDTLS_MPI_MAX_SIZE \
                                    + 5 * MPI_MAX_SIZE_2