diff --git a/benchmark/fs/readFileSync.js b/benchmark/fs/readFileSync.js
index 800ab31450f43a..bfa00070b15772 100644
--- a/benchmark/fs/readFileSync.js
+++ b/benchmark/fs/readFileSync.js
@@ -4,7 +4,7 @@ const common = require('../common.js');
 const fs = require('fs');
 
 const bench = common.createBenchmark(main, {
-  encoding: ['undefined', 'utf8'],
+  encoding: ['undefined', 'utf8', 'ascii'],
   path: ['existing', 'non-existing'],
   hasFileDescriptor: ['true', 'false'],
   n: [1e4],
diff --git a/benchmark/fs/readfile.js b/benchmark/fs/readfile.js
index 9f74508042205f..58ea110159c1d7 100644
--- a/benchmark/fs/readfile.js
+++ b/benchmark/fs/readfile.js
@@ -12,7 +12,7 @@ const filename = tmpdir.resolve(`.removeme-benchmark-garbage-${process.pid}`);
 
 const bench = common.createBenchmark(main, {
   duration: [5],
-  encoding: ['', 'utf-8'],
+  encoding: ['', 'utf-8', 'ascii'],
   len: [1024, 16 * 1024 * 1024],
   concurrent: [1, 10],
 });