--- a/mercurial/debugcommands.py	Tue Dec 14 13:12:56 2021 -0800
+++ b/mercurial/debugcommands.py	Fri Jan 07 18:42:31 2022 -0800
@@ -272,7 +272,7 @@
                             x[fn].data() for x in (pa, p1, p2)
                         ]
                         m3 = simplemerge.Merge3Text(base, local, other)
-                        ml = [l.strip() for l in m3.merge_lines()]
+                        ml = [l.strip() for l in m3.merge_lines()[0]]
                         ml.append(b"")
                     elif at > 0:
                         ml = p1[fn].data().split(b"\n")