diff --git a/tests/include/test/bignum_helpers.h b/tests/include/test/bignum_helpers.h
index ab3c86a..164017e 100644
--- a/tests/include/test/bignum_helpers.h
+++ b/tests/include/test/bignum_helpers.h
@@ -61,7 +61,8 @@
  * the length of the input. In other words, it preserves leading zeros.
  *
  * The limb array is allocated with mbedtls_calloc() and must later be
- * freed with mbedtls_free().
+ * freed with mbedtls_free(). You can do that by calling
+ * mbedtls_test_mpi_mod_modulus_free_with_limbs().
  *
  * \param[in,out] N     A modulus structure. It must be initialized, but
  *                      not set up.
@@ -74,6 +75,13 @@
                                    const char *s,
                                    mbedtls_mpi_mod_rep_selector int_rep );
 
+/** Free a modulus and its limbs.
+ *
+ * \param[in] N         A modulus structure such that there is no other
+ *                      reference to `N->p`.
+ */
+void mbedtls_test_mpi_mod_modulus_free_with_limbs( mbedtls_mpi_mod_modulus *N );
+
 /** Read an MPI from a hexadecimal string.
  *
  * Like mbedtls_mpi_read_string(), but with tighter guarantees around