diff --git a/actionbody.go b/actionbody.go
index c211bb8..740f7f3 100644
--- a/actionbody.go
+++ b/actionbody.go
@@ -36,7 +36,7 @@ type ActionBodyUntrustedData struct {
 }
 
 type ActionBodyTrustedData struct {
-	MessageBytes opt.Optional[string]
+	MessageBytes opt.Optional[string] `json:"messageBytes"`
 }
 
 type ActionBodyCastID struct {