diff --git a/revision.c b/revision.c
index 669bc85669..417659cfcb 100644
--- a/revision.c
+++ b/revision.c
@@ -2323,8 +2323,15 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 		revs->diff = 1;
 		revs->diffopt.flags.recursive = 1;
 		revs->diffopt.flags.tree_in_recursive = 1;
-	} else if (!strcmp(arg, "-m") || !strcmp(arg, "--diff-merges")) {
+	} else if (!strcmp(arg, "-m")) {
 		revs->ignore_merges = 0;
+	} else if ((argcount = parse_long_opt("diff-merges", argv, &optarg))) {
+		if (!strcmp(optarg, "off")) {
+			revs->ignore_merges = 1;
+		} else {
+			die(_("unknown value for --diff-merges: %s"), optarg);
+		}
+		return argcount;
 	} else if (!strcmp(arg, "--no-diff-merges")) {
 		revs->ignore_merges = 1;
 	} else if (!strcmp(arg, "-c")) {