diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index acdda17d84..d967d188a3 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -1004,7 +1004,9 @@ static struct base_data *find_unresolved_deltas_1(struct base_data *base,
 
 		if (!compare_and_swap_type(&child->real_type, OBJ_REF_DELTA,
 					   base->obj->real_type))
-			BUG("child->real_type != OBJ_REF_DELTA");
+			die("REF_DELTA at offset %"PRIuMAX" already resolved (duplicate base %s?)",
+			    (uintmax_t)child->idx.offset,
+			    oid_to_hex(&base->obj->idx.oid));
 
 		resolve_delta(child, base, result);
 		if (base->ref_first == base->ref_last && base->ofs_last == -1)
diff --git a/t/t5309-pack-delta-cycles.sh b/t/t5309-pack-delta-cycles.sh
index 491556dad9..6c209ad45c 100755
--- a/t/t5309-pack-delta-cycles.sh
+++ b/t/t5309-pack-delta-cycles.sh
@@ -62,13 +62,13 @@ test_expect_success 'index-pack detects REF_DELTA cycles' '
 	test_must_fail git index-pack --fix-thin --stdin <cycle.pack
 '
 
-test_expect_failure 'failover to an object in another pack' '
+test_expect_success 'failover to an object in another pack' '
 	clear_packs &&
 	git index-pack --stdin <ab.pack &&
-	git index-pack --stdin --fix-thin <cycle.pack
+	test_must_fail git index-pack --stdin --fix-thin <cycle.pack
 '
 
-test_expect_failure 'failover to a duplicate object in the same pack' '
+test_expect_success 'failover to a duplicate object in the same pack' '
 	clear_packs &&
 	{
 		pack_header 3 &&
@@ -77,7 +77,7 @@ test_expect_failure 'failover to a duplicate object in the same pack' '
 		pack_obj $A
 	} >recoverable.pack &&
 	pack_trailer recoverable.pack &&
-	git index-pack --fix-thin --stdin <recoverable.pack
+	test_must_fail git index-pack --fix-thin --stdin <recoverable.pack
 '
 
 test_done