diff --git a/broker/broker.go b/broker/broker.go
index 9b5a2430..fc1bde6a 100644
--- a/broker/broker.go
+++ b/broker/broker.go
@@ -42,7 +42,7 @@ type RawMessage []byte
 // MarshalJSON returns m as the JSON encoding of m.
 func (m *RawMessage) MarshalJSON() ([]byte, error) {
 	if m == nil {
-		return nil, nil
+		return []byte("null"), nil
 	}
 	return *m, nil
 }