diff --git a/include/polarssl/ecp.h b/include/polarssl/ecp.h
index a7f6258..55aa282 100644
--- a/include/polarssl/ecp.h
+++ b/include/polarssl/ecp.h
@@ -201,6 +201,7 @@
  *
  * \return          0 if successful,
  *                  POLARSSL_ERR_MPI_MALLOC_FAILED if memory allocation failed
+ *                  POLARSSL_ERR_ECP_GENERIC if m < 0
  */
 int ecp_mul( const ecp_group *grp, ecp_point *R,
              const mpi *m, const ecp_point *P );