diff --git a/src/pkg/encoding/json/decode.go b/src/pkg/encoding/json/decode.go
index dde0d78e327..af1c908ad77 100644
--- a/src/pkg/encoding/json/decode.go
+++ b/src/pkg/encoding/json/decode.go
@@ -54,6 +54,11 @@ import (
 // If no more serious errors are encountered, Unmarshal returns
 // an UnmarshalTypeError describing the earliest such error.
 //
+// The JSON null value unmarshals into an interface, map, pointer, or slice
+// by setting that Go value to nil. Because null is often used in JSON to mean
+// ``not present,'' unmarshaling a JSON null into any other Go type has no effect
+// on the value and produces no error.
+//
 // When unmarshaling quoted strings, invalid UTF-8 or
 // invalid UTF-16 surrogate pairs are not treated as an error.
 // Instead, they are replaced by the Unicode replacement