diff --git a/unum/vendor/golang.org/x/sys/windows/env_unset.go b/unum/vendor/golang.org/x/sys/windows/env_unset.go
new file mode 100644
index 0000000..4ed03ae
--- /dev/null
+++ b/unum/vendor/golang.org/x/sys/windows/env_unset.go
@@ -0,0 +1,15 @@
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build windows
+// +build go1.4
+
+package windows
+
+import "syscall"
+
+func Unsetenv(key string) error {
+	// This was added in Go 1.4.
+	return syscall.Unsetenv(key)
+}