diff --git a/tests/suites/test_suite_x509write.function b/tests/suites/test_suite_x509write.function
index 4de9addca..b59fd48f3 100644
--- a/tests/suites/test_suite_x509write.function
+++ b/tests/suites/test_suite_x509write.function
@@ -665,13 +665,15 @@ void x509_set_serial_check()
     mbedtls_x509write_cert ctx;
     uint8_t invalid_serial[MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN + 1];
 
+#if defined(MBEDTLS_TEST_DEPRECATED) && defined(MBEDTLS_BIGNUM_C)
+    mbedtls_mpi serial_mpi;
+    mbedtls_mpi_init(&serial_mpi);
+#endif
+
     USE_PSA_INIT();
     memset(invalid_serial, 0x01, sizeof(invalid_serial));
 
 #if defined(MBEDTLS_TEST_DEPRECATED) && defined(MBEDTLS_BIGNUM_C)
-    mbedtls_mpi serial_mpi;
-
-    mbedtls_mpi_init(&serial_mpi);
     TEST_EQUAL(mbedtls_mpi_read_binary(&serial_mpi, invalid_serial,
                                        sizeof(invalid_serial)), 0);
     TEST_EQUAL(mbedtls_x509write_crt_set_serial(&ctx, &serial_mpi),