diff --git a/lib/libsys/pdfork.2 b/lib/libsys/pdfork.2
index ad7eecb00da..9cbeb65bc4f 100644
--- a/lib/libsys/pdfork.2
+++ b/lib/libsys/pdfork.2
@@ -114,11 +114,13 @@ for a description of the possible
 flags.
 The
 .Fn pdrfork
-system call requires that the
+system call requires that both the
 .Va RFPROC
-or
+and
+.Va RFPROCDESC
+flags, or
 .Va RFSPAWN
-flag is specified.
+flag are specified.
 .Pp
 .Fn pdgetpid
 queries the process ID (PID) in the process descriptor
diff --git a/sys/kern/kern_fork.c b/sys/kern/kern_fork.c
index 6313bf3bb65..36647ae44f0 100644
--- a/sys/kern/kern_fork.c
+++ b/sys/kern/kern_fork.c
@@ -214,6 +214,11 @@ sys_pdrfork(struct thread *td, struct pdrfork_args *uap)
 		fr.fr_flags = RFFDG | RFPROC | RFPPWAIT | RFMEM | RFPROCDESC;
 		fr.fr_flags2 = FR2_DROPSIG_CAUGHT;
 	} else {
+		if ((uap->rfflags & (RFPROC | RFPROCDESC)) !=
+		    (RFPROC | RFPROCDESC)) {
+			return (EXTERROR(EINVAL,
+			    "RFPROC|RFPROCDESC required %#jx", uap->rfflags));
+		}
 		fr.fr_flags = uap->rfflags;
 	}