diff --git a/test/parallel/test-https-unix-socket-self-signed.js b/test/parallel/test-https-unix-socket-self-signed.js
index df6773f8390675..6653cdd5f8de08 100644
--- a/test/parallel/test-https-unix-socket-self-signed.js
+++ b/test/parallel/test-https-unix-socket-self-signed.js
@@ -9,8 +9,8 @@ common.refreshTmpDir();
 const fs = require('fs');
 const https = require('https');
 const options = {
-  cert: fs.readFileSync(common.fixturesDir + '/test_cert.pem'),
-  key: fs.readFileSync(common.fixturesDir + '/test_key.pem')
+  cert: fs.readFileSync(`${common.fixturesDir}/test_cert.pem`),
+  key: fs.readFileSync(`${common.fixturesDir}/test_key.pem`)
 };
 
 const server = https.createServer(options, common.mustCall((req, res) => {