diff --git a/src/os/env.go b/src/os/env.go
index 4a147144c3..aa83ee3a97 100644
--- a/src/os/env.go
+++ b/src/os/env.go
@@ -76,6 +76,7 @@ func getShellName(s string) (string, int) {
 
 // Getenv retrieves the value of the environment variable named by the key.
 // It returns the value, which will be empty if the variable is not present.
+// To distinguish between an empty value and an unset value, use LookupEnv.
 func Getenv(key string) string {
 	v, _ := syscall.Getenv(key)
 	return v