diff --git a/lib/filter.js b/lib/filter.js
index beab7f0..87297b0 100644
--- a/lib/filter.js
+++ b/lib/filter.js
@@ -115,7 +115,7 @@ function logicGzip () {
       assetPath.on('end', () => {
         if (assetTxt.length) {
           // gzip compress using highest level
-          zlib.gzip(assetTxt, { level: zlib.Z_BEST_COMPRESSION }, (err, Input) => {
+          zlib.gzip(assetTxt, { level: zlib.constants.Z_BEST_COMPRESSION }, (err, Input) => {
             if (!err) {
               // Save the compressed file to .gz
               route.set(path + '.gz', Input)