diff --git a/include/polarssl/ecp.h b/include/polarssl/ecp.h
index 4391914..e4fce69 100644
--- a/include/polarssl/ecp.h
+++ b/include/polarssl/ecp.h
@@ -444,6 +444,9 @@
  *
  * \return          0 if successful,
  *                  POLARSSL_ERR_MPI_MALLOC_FAILED if memory allocation failed
+ *
+ * \note            This function does not support Montgomery curves, such as
+ *                  Curve25519.
  */
 int ecp_add( const ecp_group *grp, ecp_point *R,
              const ecp_point *P, const ecp_point *Q );
@@ -458,6 +461,9 @@
  *
  * \return          0 if successful,
  *                  POLARSSL_ERR_MPI_MALLOC_FAILED if memory allocation failed
+ *
+ * \note            This function does not support Montgomery curves, such as
+ *                  Curve25519.
  */
 int ecp_sub( const ecp_group *grp, ecp_point *R,
              const ecp_point *P, const ecp_point *Q );