diff --git a/lib/libc/sys/open.2 b/lib/libc/sys/open.2
index 11d290628a4..73435287d16 100644
--- a/lib/libc/sys/open.2
+++ b/lib/libc/sys/open.2
@@ -268,6 +268,13 @@ and
 were specified and the file exists.
 .It Bq Er EOPNOTSUPP
 An attempt was made to open a socket (not currently implemented).
+.It Bq Er EINVAL
+An attempt was made to open a descriptor with an illegal combination
+of
+.Dv O_RDONLY ,
+.Dv O_WRONLY ,
+and
+.Dv O_RDWR .
 .El
 .Sh SEE ALSO
 .Xr chmod 2 ,