--- a/mercurial/merge.py	Sat Jul 11 00:31:21 2020 +0530
+++ b/mercurial/merge.py	Thu Jul 09 18:48:55 2020 -0700
@@ -1121,8 +1121,14 @@
     matchfiles = scmutil.matchfiles
     prefetch(
         repo,
-        [ctx.rev()],
-        matchfiles(repo, [f for sublist in oplist for f, args, msg in sublist]),
+        [
+            (
+                ctx.rev(),
+                matchfiles(
+                    repo, [f for sublist in oplist for f, args, msg in sublist]
+                ),
+            )
+        ],
     )