diff --git a/src/expvar/expvar.go b/src/expvar/expvar.go
index d675a0230cd..1ec85006b4d 100644
--- a/src/expvar/expvar.go
+++ b/src/expvar/expvar.go
@@ -38,6 +38,7 @@ import (
 
 // Var is an abstract type for all exported variables.
 type Var interface {
+	// String returns a valid JSON value for the variable.
 	String() string
 }