diff --git a/tests/suites/test_suite_ecp.function b/tests/suites/test_suite_ecp.function
index 212dfcbf9..4e74d9b8e 100644
--- a/tests/suites/test_suite_ecp.function
+++ b/tests/suites/test_suite_ecp.function
@@ -1375,6 +1375,7 @@ void ecp_mod_p521_raw(char *input_N,
                    MBEDTLS_MPI_MOD_REP_MONTGOMERY), 0);
 
     TEST_EQUAL(mbedtls_ecp_mod_p521_raw(X, limbs_X), 0);
+    TEST_LE_U(mbedtls_mpi_core_bitlen(X, limbs_X), 522);
     mbedtls_mpi_mod_raw_fix_quasi_reduction(X, &m);
     ASSERT_COMPARE(X, bytes, res, bytes);