diff --git a/jsonpb.go b/jsonpb.go
index 7fb0653..f966b70 100644
--- a/jsonpb.go
+++ b/jsonpb.go
@@ -39,16 +39,17 @@ func (c *jsonpbCodec) Marshal(v interface{}, opts ...codec.Option) ([]byte, erro
 	case *codec.Frame:
 		return m.Data, nil
 	case proto.Message:
-		if nv, nerr := rutil.StructFieldByTag(m, codec.DefaultTagName, flattenTag); nerr == nil {
-			if nm, ok := nv.(proto.Message); ok {
-				m = nm
-			}
-		}
 		options := c.opts
 		for _, o := range opts {
 			o(&options)
 		}
 
+		if nv, nerr := rutil.StructFieldByTag(m, options.TagName, flattenTag); nerr == nil {
+			if nm, ok := nv.(proto.Message); ok {
+				m = nm
+			}
+		}
+
 		marshalOptions := DefaultMarshalOptions
 		if options.Context != nil {
 			if f, ok := options.Context.Value(marshalOptionsKey{}).(jsonpb.MarshalOptions); ok {
@@ -72,17 +73,17 @@ func (c *jsonpbCodec) Unmarshal(d []byte, v interface{}, opts ...codec.Option) e
 		m.Data = d
 		return nil
 	case proto.Message:
-		if nv, nerr := rutil.StructFieldByTag(m, codec.DefaultTagName, flattenTag); nerr == nil {
-			if nm, ok := nv.(proto.Message); ok {
-				m = nm
-			}
-		}
-
 		options := c.opts
 		for _, o := range opts {
 			o(&options)
 		}
 
+		if nv, nerr := rutil.StructFieldByTag(m, options.TagName, flattenTag); nerr == nil {
+			if nm, ok := nv.(proto.Message); ok {
+				m = nm
+			}
+		}
+
 		unmarshalOptions := DefaultUnmarshalOptions
 		if options.Context != nil {
 			if f, ok := options.Context.Value(unmarshalOptionsKey{}).(jsonpb.UnmarshalOptions); ok {