diff --git a/builtin/rebase.c b/builtin/rebase.c
index dd31d5ab91..4b3f29a449 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -1141,7 +1141,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		{OPTION_STRING, 'r', "rebase-merges", &rebase_merges,
 			N_("mode"),
 			N_("try to rebase merges instead of skipping them"),
-			PARSE_OPT_OPTARG, NULL, (intptr_t)""},
+			PARSE_OPT_OPTARG, NULL, (intptr_t)"no-rebase-cousins"},
 		OPT_BOOL(0, "fork-point", &options.fork_point,
 			 N_("use 'merge-base --fork-point' to refine upstream")),
 		OPT_STRING('s', "strategy", &options.strategy,
@@ -1439,7 +1439,11 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 
 	if (rebase_merges) {
 		if (!*rebase_merges)
-			; /* default mode; do nothing */
+			warning(_("--rebase-merges with an empty string "
+				  "argument is deprecated and will stop "
+				  "working in a future version of Git. Use "
+				  "--rebase-merges without an argument "
+				  "instead, which does the same thing."));
 		else if (!strcmp("rebase-cousins", rebase_merges))
 			options.rebase_cousins = 1;
 		else if (strcmp("no-rebase-cousins", rebase_merges))