diff --git a/library/ecp_invasive.h b/library/ecp_invasive.h
index 45b0006963..3d1321c525 100644
--- a/library/ecp_invasive.h
+++ b/library/ecp_invasive.h
@@ -99,12 +99,21 @@ int mbedtls_ecp_mod_p192_raw(mbedtls_mpi_uint *Np, size_t Nn);
 
 /** Fast quasi-reduction modulo p521 = 2^521 - 1 (FIPS 186-3 D.2.5)
  *
- * \param[in,out]   N_p     The address of the MPI to be converted.
- *                          Must have 2 * N - 1 limbs, where N is the modulus.
- * \param[in]       N_n     The length of \p N_p in limbs.
+ * \param[in,out]   X       The address of the MPI to be converted.
+ *                          Must have twice as many limbs as the modulus
+ *                          (the modulus is 521 bits long). Upon return this
+ *                          holds the reduced value. The reduced value is
+ *                          in range `0 <= X < 2 * N` (where N is the modulus).
+ *                          and its the bitlength is one plus the bitlength
+ *                          of the modulus.
+ * \param[in]       X_limbs The length of \p X in limbs.
+ *
+ * \return          \c 0 on success.
+ * \return          #MBEDTLS_ERR_ECP_BAD_INPUT_DATA if \p X_limbs does not have
+ *                  twice as many limbs as the modulus.
  */
 MBEDTLS_STATIC_TESTABLE
-int mbedtls_ecp_mod_p521_raw(mbedtls_mpi_uint *N_p, size_t N_n);
+int mbedtls_ecp_mod_p521_raw(mbedtls_mpi_uint *X, size_t X_limbs);
 
 #endif /* MBEDTLS_ECP_DP_SECP521R1_ENABLED */