diff --git a/flask/helpers.py b/flask/helpers.py
index 9bcb22bb..8780ee20 100644
--- a/flask/helpers.py
+++ b/flask/helpers.py
@@ -571,7 +571,7 @@ def send_file(filename_or_fp, mimetype=None, as_attachment=False,
             os.path.getmtime(filename),
             os.path.getsize(filename),
             adler32(
-                filename.encode('utf8') if isinstance(filename, unicode)
+                filename.encode('utf-8') if isinstance(filename, unicode)
                 else filename
             ) & 0xffffffff
         ))