diff --git a/youtube_dl/YoutubeDL.py b/youtube_dl/YoutubeDL.py
index 925d095de..523dd1f7d 100755
--- a/youtube_dl/YoutubeDL.py
+++ b/youtube_dl/YoutubeDL.py
@@ -1033,7 +1033,7 @@ def _build_format_filter(self, filter_spec):
             '!=': operator.ne,
         }
         operator_rex = re.compile(r'''(?x)\s*
-            (?P<key>width|height|tbr|abr|vbr|asr|filesize|fps)
+            (?P<key>width|height|tbr|abr|vbr|asr|filesize|filesize_approx|fps)
             \s*(?P<op>%s)(?P<none_inclusive>\s*\?)?\s*
             (?P<value>[0-9.]+(?:[kKmMgGtTpPeEzZyY]i?[Bb]?)?)
             $