diff --git a/Sources/SystemLibrary.swift b/Sources/SystemLibrary.swift
index d51e046..1cb83a0 100644
--- a/Sources/SystemLibrary.swift
+++ b/Sources/SystemLibrary.swift
@@ -114,7 +114,7 @@ struct SystemLibrary {
         static func fdSet(fd: Int32, set: inout fd_set) {
             let intOffset = Int(fd / 32)
             let bitOffset = fd % 32
-            let mask = Int32(1 << bitOffset)
+            let mask = Int32((1 << bitOffset) * (bitOffset == 31 ? -1 : 1))
             switch intOffset {
             case 0: set.fds_bits.0 = set.fds_bits.0 | mask
             case 1: set.fds_bits.1 = set.fds_bits.1 | mask
@@ -155,7 +155,7 @@ struct SystemLibrary {
         static func fdIsSet(fd: Int32, set: inout fd_set) -> Bool {
             let intOffset = Int(fd / 32)
             let bitOffset = fd % 32
-            let mask = Int32(1 << bitOffset)
+            let mask = Int32((1 << bitOffset) * (bitOffset == 31 ? -1 : 1))
             switch intOffset {
             case 0: return set.fds_bits.0 & mask != 0
             case 1: return set.fds_bits.1 & mask != 0