diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index b8cfb1328bbfc0..34c64df2c6ccf8 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -1741,6 +1741,7 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 	unsigned foreign_nr = 1;	/* zero is a "good" value, assume bad */
 	int report_end_of_input = 0;
 	int hash_algo = 0;
+	int dash_o = 0;
 
 	/*
 	 * index-pack never needs to fetch missing objects except when
@@ -1833,6 +1834,7 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 				if (index_name || (i+1) >= argc)
 					usage(index_pack_usage);
 				index_name = argv[++i];
+				dash_o = 1;
 			} else if (starts_with(arg, "--index-version=")) {
 				char *c;
 				opts.version = strtoul(arg + 16, &c, 10);
@@ -1875,6 +1877,8 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 		index_name = derive_filename(pack_name, "pack", "idx", &index_name_buf);
 
 	opts.flags &= ~(WRITE_REV | WRITE_REV_VERIFY);
+	if (rev_index && dash_o && !ends_with(index_name, ".idx"))
+		rev_index = 0;
 	if (rev_index) {
 		opts.flags |= verify ? WRITE_REV_VERIFY : WRITE_REV;
 		if (index_name)
diff --git a/t/t5300-pack-object.sh b/t/t5300-pack-object.sh
index 648cb177512e41..70013c5a090863 100755
--- a/t/t5300-pack-object.sh
+++ b/t/t5300-pack-object.sh
@@ -372,11 +372,10 @@ test_expect_success 'complain about index name' '
 	test -f test-complain-0.idx &&
 	test -f test-complain-0.rev &&
 
-	# Non .idx suffix
+	# Non .idx suffix -- implicitly omits the .rev
 	cat test-1-${packname_1}.pack >test-complain-1.pack &&
-	test_must_fail git index-pack -o test-complain-1.idx-suffix --rev-index test-complain-1.pack 2>err &&
-	grep "does not end" err &&
-	! test -f test-complain-1.idx-suffix &&
+	git index-pack -o test-complain-1.idx-suffix --rev-index test-complain-1.pack &&
+	test -f test-complain-1.idx-suffix &&
 	! test -f test-complain-1.rev
 '