diff --git a/lfs.c b/lfs.c
index 8ce1872..f7235f6 100644
--- a/lfs.c
+++ b/lfs.c
@@ -92,6 +92,7 @@ static int lfs_bd_read(lfs_t *lfs,
                 lfs->cfg->cache_size);
         int err = lfs->cfg->read(lfs->cfg, rcache->block,
                 rcache->off, rcache->buffer, rcache->size);
+        LFS_ASSERT(err <= 0);
         if (err) {
             return err;
         }
@@ -136,6 +137,7 @@ static int lfs_bd_flush(lfs_t *lfs,
         lfs_size_t diff = lfs_alignup(pcache->size, lfs->cfg->prog_size);
         int err = lfs->cfg->prog(lfs->cfg, pcache->block,
                 pcache->off, pcache->buffer, diff);
+        LFS_ASSERT(err <= 0);
         if (err) {
             return err;
         }
@@ -170,7 +172,9 @@ static int lfs_bd_sync(lfs_t *lfs,
         return err;
     }
 
-    return lfs->cfg->sync(lfs->cfg);
+    err = lfs->cfg->sync(lfs->cfg);
+    LFS_ASSERT(err <= 0);
+    return err;
 }
 
 static int lfs_bd_prog(lfs_t *lfs,
@@ -221,7 +225,9 @@ static int lfs_bd_prog(lfs_t *lfs,
 
 static int lfs_bd_erase(lfs_t *lfs, lfs_block_t block) {
     LFS_ASSERT(block < lfs->cfg->block_count);
-    return lfs->cfg->erase(lfs->cfg, block);
+    int err = lfs->cfg->erase(lfs->cfg, block);
+    LFS_ASSERT(err <= 0);
+    return err;
 }