int saved_nrl = 0;
                int saved_dcctc = 0;
 
-               if (opt->diffopt.detect_rename)
-                       opt->diffopt.setup |= (DIFF_SETUP_USE_SIZE_CACHE |
-                                              DIFF_SETUP_USE_CACHE);
+               if (opt->diffopt.detect_rename) {
+                       if (!the_index.cache)
+                               read_index(&the_index);
+                       opt->diffopt.setup |= DIFF_SETUP_USE_SIZE_CACHE;
+               }
                while (fgets(line, sizeof(line), stdin)) {
                        struct object_id oid;