diff --git a/mrbgems/hal-posix-io/src/io_hal.c b/mrbgems/hal-posix-io/src/io_hal.c
index bba87ce33..2283fdd7e 100644
--- a/mrbgems/hal-posix-io/src/io_hal.c
+++ b/mrbgems/hal-posix-io/src/io_hal.c
@@ -519,6 +519,10 @@ void
 mrb_hal_io_fdset_set(mrb_state *mrb, int fd, mrb_io_fdset *fdset)
 {
   (void)mrb;
+  if (fd < 0 || fd >= FD_SETSIZE) {
+    mrb_raise(mrb, E_ARGUMENT_ERROR, "fd is out of range");
+    return;
+  }
   if (fdset) {
     FD_SET(fd, &fdset->fds);
   }
@@ -528,6 +532,10 @@ int
 mrb_hal_io_fdset_isset(mrb_state *mrb, int fd, mrb_io_fdset *fdset)
 {
   (void)mrb;
+  if (fd < 0 || fd >= FD_SETSIZE) {
+    mrb_raise(mrb, E_ARGUMENT_ERROR, "fd is out of range");
+    return 0;
+  }
   if (fdset) {
     return FD_ISSET(fd, &fdset->fds);
   }
diff --git a/mrbgems/hal-win-io/src/io_hal.c b/mrbgems/hal-win-io/src/io_hal.c
index 9cc3583bd..f3429b62a 100644
--- a/mrbgems/hal-win-io/src/io_hal.c
+++ b/mrbgems/hal-win-io/src/io_hal.c
@@ -562,6 +562,11 @@ void
 mrb_hal_io_fdset_set(mrb_state *mrb, int fd, mrb_io_fdset *fdset)
 {
   (void)mrb;
+  if (fdset->fds.fd_count >= FD_SETSIZE) {
+      mrb_raise(mrb, E_ARGUMENT_ERROR, "too many sockets for fd_set");
+      return;
+  }
+
   if (fdset) {
     FD_SET(fd, &fdset->fds);
   }