diff --git a/mercurial/patch.py b/mercurial/patch.py
--- a/mercurial/patch.py
+++ b/mercurial/patch.py
@@ -2330,6 +2330,11 @@ def diff(repo, node1=None, node2=None, m
             losedatafn=losedatafn, prefix=prefix, relroot=relroot, copy=copy,
     ):
         if hunksfilterfn is not None:
+            # If the file has been removed, fctx2 is None; but this should
+            # not occur here since we catch removed files early in
+            # cmdutil.getloglinerangerevs() for 'hg log -L'.
+            assert fctx2 is not None, \
+                'fctx2 unexpectly None in diff hunks filtering'
             hunks = hunksfilterfn(fctx2, hunks)
         text = ''.join(sum((list(hlines) for hrange, hlines in hunks), []))
         if hdr and (text or len(hdr) > 1):