diff --git a/lfs.c b/lfs.c
index 4086f00e..967e7c42 100644
--- a/lfs.c
+++ b/lfs.c
@@ -93,6 +93,7 @@ static int lfs_bd_read(lfs_t *lfs,
             // bypass cache?
             diff = lfs_aligndown(diff, lfs->cfg->read_size);
             int err = lfs->cfg->read(lfs->cfg, block, off, data, diff);
+            LFS_ASSERT(err <= 0);
             if (err) {
                 return err;
             }