diff --git a/codec.go b/codec.go
index eb1b09c..db9706d 100644
--- a/codec.go
+++ b/codec.go
@@ -140,7 +140,7 @@ func (g *grpcCodec) ReadHeader(m *codec.Message, mt codec.MessageType) error {
 		m = new(codec.Message)
 	}
 	if m.Header == nil {
-		m.Header = make(map[string]string)
+		m.Header = make(map[string]string, len(md))
 	}
 	for k, v := range md {
 		m.Header[k] = strings.Join(v, ",")
diff --git a/subscriber.go b/subscriber.go
index 7bd39a5..a29ebcb 100644
--- a/subscriber.go
+++ b/subscriber.go
@@ -188,7 +188,7 @@ func (g *grpcServer) createSubHandler(sb *subscriber, opts server.Options) broke
 			return err
 		}
 
-		hdr := make(map[string]string)
+		hdr := make(map[string]string, len(msg.Header))
 		for k, v := range msg.Header {
 			hdr[k] = v
 		}