diff --git a/lib/buffer.js b/lib/buffer.js
index a68718848c1a49..05b57275f03dca 100644
--- a/lib/buffer.js
+++ b/lib/buffer.js
@@ -782,13 +782,22 @@ function byteLength(string, encoding) {
   if (len === 0)
     return 0;
 
-  if (encoding) {
-    const ops = getEncodingOps(encoding);
-    if (ops) {
-      return ops.byteLength(string);
-    }
+  if (!encoding || encoding === 'utf8') {
+    return byteLengthUtf8(string);
+  }
+
+  if (encoding === 'ascii') {
+    return len;
   }
-  return byteLengthUtf8(string);
+
+  const ops = getEncodingOps(encoding);
+  if (ops === undefined) {
+    // TODO (ronag): Makes more sense to throw here.
+    // throw new ERR_UNKNOWN_ENCODING(encoding);
+    return byteLengthUtf8(string);
+  }
+
+  return ops.byteLength(string);
 }
 
 Buffer.byteLength = byteLength;