diff --git a/changelog.md b/changelog.md
index 0937dde..a61f36a 100644
--- a/changelog.md
+++ b/changelog.md
@@ -22,6 +22,7 @@
   - Updated `actions/setup-node` to v3.
 - Reorganized the test file structure.
 - Use the `.js` file extension in `require` paths.
+- Use the Node.js `Readable` property `readableEncoding` instead of `_readableState.encoding` in tests.
 - Fixed a typo in a code comment.
 
 ## 13.0.0
diff --git a/processRequest.test.mjs b/processRequest.test.mjs
index adcd661..02ba1fa 100644
--- a/processRequest.test.mjs
+++ b/processRequest.test.mjs
@@ -67,7 +67,7 @@ export default (tests) => {
           const stream = upload.createReadStream();
 
           ok(stream instanceof ReadStream);
-          strictEqual(stream._readableState.encoding, null);
+          strictEqual(stream.readableEncoding, null);
           strictEqual(stream.readableHighWaterMark, 16384);
           strictEqual(await streamToString(stream), "a");
         } catch (error) {
@@ -120,7 +120,7 @@ export default (tests) => {
           const stream = upload.createReadStream({ encoding, highWaterMark });
 
           ok(stream instanceof ReadStream);
-          strictEqual(stream._readableState.encoding, encoding);
+          strictEqual(stream.readableEncoding, encoding);
           strictEqual(stream.readableHighWaterMark, highWaterMark);
           strictEqual(
             await streamToString(stream),