diff --git a/src/os/file_windows.go b/src/os/file_windows.go
index 9191830144..8ace9c0c7f 100644
--- a/src/os/file_windows.go
+++ b/src/os/file_windows.go
@@ -87,6 +87,8 @@ type dirInfo struct {
 func epipecheck(file *File, e error) {
 }
 
+// DevNull is the name of the operating system's ``null device.''
+// On Unix-like systems, it is "/dev/null"; on Windows, "NUL".
 const DevNull = "NUL"
 
 func (f *file) isdir() bool { return f != nil && f.dirinfo != nil }