diff --git a/test/common/tmpdir.js b/test/common/tmpdir.js
index ced957e5547bb9..33b2264a8d69f5 100644
--- a/test/common/tmpdir.js
+++ b/test/common/tmpdir.js
@@ -89,11 +89,7 @@ const testRoot = process.env.NODE_TEST_DIR ?
 
 // Using a `.` prefixed name, which is the convention for "hidden" on POSIX,
 // gets tools to ignore it by default or by simple rules, especially eslint.
-let tmpdirName = '.tmp';
-if (process.env.TEST_THREAD_ID) {
-  tmpdirName += `.${process.env.TEST_THREAD_ID}`;
-}
-
+const tmpdirName = '.tmp.' + (process.env.TEST_THREAD_ID || '0');
 const tmpPath = path.join(testRoot, tmpdirName);
 
 function refresh(opts = {}) {