diff --git a/proto/encode.go b/proto/encode.go
index 8635790..1b48b41 100644
--- a/proto/encode.go
+++ b/proto/encode.go
@@ -184,7 +184,7 @@
 		b = wire.AppendTag(b, num, wireTypes[kind])
 		return o.marshalSingular(b, num, field, value)
 	case field.IsMap():
-		return o.marshalMap(b, num, kind, field.MessageType(), value.Map())
+		return o.marshalMap(b, num, kind, field.Message(), value.Map())
 	case field.IsPacked():
 		return o.marshalPacked(b, num, field, value.List())
 	default: