#include "builtin.h"
 
 static const char diff_cache_usage[] =
-"git-diff-index [-m] [--cached] "
+"git diff-index [-m] [--cached] "
 "[<common diff options>] <tree-ish> [<path>...]"
 COMMON_DIFF_OPTIONS_HELP;
 
        if (rev.pending.nr != 1 ||
            rev.max_count != -1 || rev.min_age != -1 || rev.max_age != -1)
                usage(diff_cache_usage);
+       if (!cached)
+               setup_work_tree();
        if (read_cache() < 0) {
                perror("read_cache");
                return -1;