diff -r a4a95bd7158d -r a82fc3922446 mercurial/hgweb/webcommands.py
--- a/mercurial/hgweb/webcommands.py	Fri Jan 19 18:52:50 2018 -0800
+++ b/mercurial/hgweb/webcommands.py	Fri Mar 16 12:10:15 2018 -0700
@@ -1195,7 +1195,9 @@
         web.res.headers['Content-Encoding'] = encoding
 
     web.res.setbodywillwrite()
-    assert list(web.res.sendresponse()) == []
+    if list(web.res.sendresponse()):
+        raise error.ProgrammingError('sendresponse() should not emit data '
+                                     'if writing later')
 
     bodyfh = web.res.getbodyfile()