diff --git a/man2/sigaction.2 b/man2/sigaction.2
index c9cb18de0..ce59c9399 100644
--- a/man2/sigaction.2
+++ b/man2/sigaction.2
@@ -210,6 +210,7 @@ is a struct with the following elements
 .nf
 siginfo_t {
 .\" FIXME si_tid and si_overrun are not documented.
+.\" FIXME si_trapno is not documented; is it actually used?
     int      si_signo;    /* Signal number */
     int      si_errno;    /* An errno value */
     int      si_code;     /* Signal code */