diff --git a/doc/man3/BIO_sendmmsg.pod b/doc/man3/BIO_sendmmsg.pod
index 9742eda50b..04075dd591 100644
--- a/doc/man3/BIO_sendmmsg.pod
+++ b/doc/man3/BIO_sendmmsg.pod
@@ -197,6 +197,10 @@ transient error, many of which are system specific.
 
 =back
 
+Third parties implementing custom BIOs supporting the BIO_sendmmsg() or
+BIO_recvmmsg() methods should note that it is a required part of the API
+contract that an error is always raised when either of these functions return 0.
+
 BIO_dgram_set_local_addr_enable() returns 1 if local address support was
 successfully enabled or disabled and 0 otherwise.