diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 1f6ea9c1d..91ce2afcd 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -42,7 +42,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [macOS-latest, windows-latest, ubuntu-latest]
-        node-version: [14, 16, 18, 19, 20]
+        node-version: [14, 16, 18, 19, 20, 21]
         exclude:
           - os: windows-latest
             node-version: 14
diff --git a/lib/proto.js b/lib/proto.js
index 624f809ab..b82fd0c26 100644
--- a/lib/proto.js
+++ b/lib/proto.js
@@ -226,7 +226,7 @@ function flush (cb) {
 
   const stream = this[streamSym]
 
-  if ('flush' in stream) {
+  if (typeof stream.flush === 'function') {
     stream.flush(cb || noop)
   } else if (cb) cb()
 }