diff --git a/volumes.c b/volumes.c
index 77ffd325..b39f3743 100644
--- a/volumes.c
+++ b/volumes.c
@@ -630,7 +630,8 @@ int btrfs_add_system_chunk(struct btrfs_trans_handle *trans,
 	u8 *ptr;
 
 	array_size = btrfs_super_sys_array_size(super_copy);
-	if (array_size + item_size > BTRFS_SYSTEM_CHUNK_ARRAY_SIZE)
+	if (array_size + item_size + sizeof(disk_key)
+			> BTRFS_SYSTEM_CHUNK_ARRAY_SIZE)
 		return -EFBIG;
 
 	ptr = super_copy->sys_chunk_array + array_size;