diff --git a/library/ecp_invasive.h b/library/ecp_invasive.h
index 10aa34b..aba7cca 100644
--- a/library/ecp_invasive.h
+++ b/library/ecp_invasive.h
@@ -118,6 +118,23 @@
 
 #endif /* MBEDTLS_ECP_DP_SECP521R1_ENABLED */
 
+/** Initialise a modulus with hard-coded const curve data.
+ *
+ * \note            The caller is responsible for the \p N modulus' memory.
+ *                  mbedtls_mpi_mod_modulus_free(&N) should be invoked at the
+ *                  end of its lifecycle.
+ *
+ * \param[in,out] N The address of the modulus structure to populate.
+ *                  Must be initialized.
+ * \param[in] id    The mbedtls_ecp_group_id for which to initialise the modulus.
+ * \param[in] ctype The mbedtls_ecp_curve_type identifier for a coordinate modulus (P)
+ *                  or a scalar modulus (N).
+ *
+ * \return          \c 0 if successful.
+ * \return          #MBEDTLS_ERR_ECP_BAD_INPUT_DATA if the given MPIs do not
+ *                  have the correct number of limbs.
+ *
+ */
 MBEDTLS_STATIC_TESTABLE
 int mbedtls_ecp_modulus_setup(mbedtls_mpi_mod_modulus *N,
                               const mbedtls_ecp_group_id id,