diff --git a/codec.go b/codec.go
index ff37769..0366675 100644
--- a/codec.go
+++ b/codec.go
@@ -155,7 +155,7 @@ 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/grpc.go b/grpc.go
index 519b445..0905eac 100644
--- a/grpc.go
+++ b/grpc.go
@@ -110,13 +110,18 @@ func (g *grpcClient) next(request client.Request, opts client.CallOptions) (sele
 }
 
 func (g *grpcClient) call(ctx context.Context, node *registry.Node, req client.Request, rsp interface{}, opts client.CallOptions) error {
+	var header map[string]string
+
 	address := node.Address
 
-	header := make(map[string]string)
+	header = make(map[string]string)
 	if md, ok := metadata.FromContext(ctx); ok {
+		header = make(map[string]string, len(md))
 		for k, v := range md {
 			header[k] = v
 		}
+	} else {
+		header = make(map[string]string)
 	}
 
 	// set timeout in nanoseconds
@@ -182,13 +187,17 @@ func (g *grpcClient) call(ctx context.Context, node *registry.Node, req client.R
 }
 
 func (g *grpcClient) stream(ctx context.Context, node *registry.Node, req client.Request, opts client.CallOptions) (client.Stream, error) {
+	var header map[string]string
+
 	address := node.Address
 
-	header := make(map[string]string)
 	if md, ok := metadata.FromContext(ctx); ok {
+		header = make(map[string]string, len(md))
 		for k, v := range md {
 			header[k] = v
 		}
+	} else {
+		header = make(map[string]string)
 	}
 
 	// set timeout in nanoseconds
diff --git a/response.go b/response.go
index 5fd4016..cd4d319 100644
--- a/response.go
+++ b/response.go
@@ -27,7 +27,7 @@ func (r *response) Header() map[string]string {
 	if err != nil {
 		return map[string]string{}
 	}
-	hdr := make(map[string]string)
+	hdr := make(map[string]string, len(md))
 	for k, v := range md {
 		hdr[k] = strings.Join(v, ",")
 	}