argv = xrealloc(argv, sizeof(*argv) * (argc + 2));
                memmove(argv + 1, argv, sizeof(*argv) * (argc + 1));
                argc++;
-               parse_options(argc, argv, builtin_merge_options,
+               parse_options(argc, argv, NULL, builtin_merge_options,
                              builtin_merge_usage, 0);
                free(buf);
        }
 
        fp = fopen(git_path("MERGE_MSG"), "a");
        if (!fp)
-               die("Could open %s for writing", git_path("MERGE_MSG"));
+               die("Could not open %s for writing", git_path("MERGE_MSG"));
        fprintf(fp, "\nConflicts:\n");
        for (pos = 0; pos < active_nr; pos++) {
                struct cache_entry *ce = active_cache[pos];
        struct commit_list **remotes = &remoteheads;
 
        setup_work_tree();
+       if (file_exists(git_path("MERGE_HEAD")))
+               die("You have not concluded your merge. (MERGE_HEAD exists)");
        if (read_cache_unmerged())
-               die("You are in the middle of a conflicted merge.");
+               die("You are in the middle of a conflicted merge."
+                               " (index unmerged)");
 
        /*
         * Check if we are _not_ on a detached HEAD, i.e. if there is a
        if (diff_use_color_default == -1)
                diff_use_color_default = git_use_color_default;
 
-       argc = parse_options(argc, argv, builtin_merge_options,
+       argc = parse_options(argc, argv, prefix, builtin_merge_options,
                        builtin_merge_usage, 0);
        if (verbosity < 0)
                show_diffstat = 0;