diff --git a/test/parallel/test-buffer-tostring-rangeerror.js b/test/parallel/test-buffer-tostring-rangeerror.js
index b34f4c61261..856ed84d21c 100644
--- a/test/parallel/test-buffer-tostring-rangeerror.js
+++ b/test/parallel/test-buffer-tostring-rangeerror.js
@@ -34,6 +34,12 @@ function test(getBuffer) {
       return;
     }
   }
+  // MAX_STRING_LENGTH counts UTF-16 units, not bytes. Uninitialized memory can
+  // hold multi-byte UTF-8 sequences, which decode to fewer units than they
+  // occupy bytes, and a single one of them is enough to bring the result down
+  // to MAX_STRING_LENGTH or less. ASCII bytes keep the decoded length equal to
+  // the buffer length.
+  buf.fill(0);
   assert.throws(() => { buf.toString('utf8'); }, message);
 }