diff --git a/src/library/parallel/src/fork.c b/src/library/parallel/src/fork.c
index 5f3d29cdec1..f4aef9258a7 100644
--- a/src/library/parallel/src/fork.c
+++ b/src/library/parallel/src/fork.c
@@ -1,7 +1,7 @@
 /*
  *  R : A Computer Language for Statistical Data Analysis
  *  (C) Copyright 2008-2011 Simon Urbanek
- *      Copyright 2011-2022 R Core Team.
+ *      Copyright 2011-2023 R Core Team.
  *
  *  This program is free software; you can redistribute it and/or modify
  *  it under the terms of the GNU General Public License as published by
@@ -816,6 +816,8 @@ SEXP mc_send_child_stdin(SEXP sPid, SEXP what)
 void fdcopy(fd_set *dst, fd_set *src, int nfds)
 {
     FD_ZERO(dst);
+    if (nfds > FD_SETSIZE)
+	error("file descriptor is too large for select()");
     for(int i = 0; i < nfds; i++)
 	if (FD_ISSET(i, src)) FD_SET(i, dst);
 }
@@ -847,7 +849,7 @@ SEXP mc_select_children(SEXP sTimeout, SEXP sWhich)
 		unsigned int k = 0;
 		while (k < wlen) {
 		    if (which[k++] == ci->pid) {
-			if (ci->pfd > FD_SETSIZE)
+			if (ci->pfd >= FD_SETSIZE)
 			    error("file descriptor is too large for select()");
 			FD_SET(ci->pfd, &fs);
 			if (ci->pfd > maxfd) maxfd = ci->pfd;
@@ -860,7 +862,7 @@ SEXP mc_select_children(SEXP sTimeout, SEXP sWhich)
 		}
 	    } else {
 		/* not sure if this should be allowed */
-		if (ci->pfd > FD_SETSIZE)
+		if (ci->pfd >= FD_SETSIZE)
 		    error("file descriptor is too large for select()");
 		FD_SET(ci->pfd, &fs);
 		if (ci->pfd > maxfd) maxfd = ci->pfd;
@@ -1061,7 +1063,11 @@ SEXP mc_read_children(SEXP sTimeout)
     while (ci) {
 	if (!ci->detached && ci->ppid == ppid) {
 	    if (ci->pfd > maxfd) maxfd = ci->pfd;
-	    if (ci->pfd >= 0) FD_SET(ci->pfd, &fs);
+	    if (ci->pfd >= 0) {
+		if (ci->pfd >= FD_SETSIZE)
+		    error("file descriptor is too large for select()");
+		FD_SET(ci->pfd, &fs);
+	    }
 	}
 	ci = ci -> next;
     }
diff --git a/src/unix/sys-std.c b/src/unix/sys-std.c
index c0f98f14c3a..3d355238e76 100644
--- a/src/unix/sys-std.c
+++ b/src/unix/sys-std.c
@@ -120,7 +120,7 @@ int R_SelectEx(int  n,  fd_set  *readfds,  fd_set  *writefds,
        platforms. If this still turns out to be limiting we will
        probably need to rewrite internals to use poll() instead of
        select().  LT */
-    if (n > FD_SETSIZE)
+    if (n >= FD_SETSIZE)
 	error("file descriptor is too large for select()");
 
     if (timeout != NULL && timeout->tv_sec == 0 && timeout->tv_usec == 0)
@@ -229,6 +229,8 @@ addInputHandler(InputHandler *handlers, int fd, InputHandlerProc handler,
     input = R_Calloc(1, InputHandler);
 
     input->activity = activity;
+    if (fd >= FD_SETSIZE)
+	error("file descriptor is too large for select()");
     input->fileDescriptor = fd;
     input->handler = handler;
 
@@ -382,8 +384,11 @@ setSelectMask(InputHandler *handlers, fd_set *readMask)
     FD_ZERO(readMask);
 
     /* If we are dealing with BasicInputHandler always put stdin */
-    if(handlers == &BasicInputHandler)
+    if(handlers == &BasicInputHandler) {
 	handlers->fileDescriptor = fileno(stdin);
+	if (handlers->fileDescriptor >= FD_SETSIZE)
+	    error("file descriptor is too large for select()");
+    }
 
     while(tmp) {
 	FD_SET(tmp->fileDescriptor, readMask);