diff --git a/bd/lfs_emubd.c b/bd/lfs_emubd.c
index 5ab8b082..de3075ec 100644
--- a/bd/lfs_emubd.c
+++ b/bd/lfs_emubd.c
@@ -220,7 +220,9 @@ int lfs_emubd_read(const struct lfs_config *cfg, lfs_block_t block,
     lfs_emubd_t *bd = cfg->context;
 
     // check if read is valid
-    LFS_ASSERT(block < bd->cfg->erase_count);
+    if (block >= bd->cfg->erase_count) {
+        return LFS_ERR_INVAL;
+    }
     LFS_ASSERT(off  % bd->cfg->read_size == 0);
     LFS_ASSERT(size % bd->cfg->read_size == 0);
     LFS_ASSERT(off+size <= bd->cfg->erase_size);
diff --git a/bd/lfs_filebd.c b/bd/lfs_filebd.c
index 4ff25d44..5f874531 100644
--- a/bd/lfs_filebd.c
+++ b/bd/lfs_filebd.c
@@ -70,7 +70,9 @@ int lfs_filebd_read(const struct lfs_config *cfg, lfs_block_t block,
     lfs_filebd_t *bd = cfg->context;
 
     // check if read is valid
-    LFS_ASSERT(block < bd->cfg->erase_count);
+    if (block >= bd->cfg->erase_count) {
+        return LFS_ERR_INVAL;
+    }
     LFS_ASSERT(off  % bd->cfg->read_size == 0);
     LFS_ASSERT(size % bd->cfg->read_size == 0);
     LFS_ASSERT(off+size <= bd->cfg->erase_size);
diff --git a/bd/lfs_rambd.c b/bd/lfs_rambd.c
index a6a05727..1b29e336 100644
--- a/bd/lfs_rambd.c
+++ b/bd/lfs_rambd.c
@@ -60,7 +60,9 @@ int lfs_rambd_read(const struct lfs_config *cfg, lfs_block_t block,
     lfs_rambd_t *bd = cfg->context;
 
     // check if read is valid
-    LFS_ASSERT(block < bd->cfg->erase_count);
+    if (block >= bd->cfg->erase_count) {
+        return LFS_ERR_INVAL;
+    }
     LFS_ASSERT(off  % bd->cfg->read_size == 0);
     LFS_ASSERT(size % bd->cfg->read_size == 0);
     LFS_ASSERT(off+size <= bd->cfg->erase_size);
diff --git a/benches/bench_superblock.toml b/benches/bench_superblock.toml
index 37659d47..ff013700 100644
--- a/benches/bench_superblock.toml
+++ b/benches/bench_superblock.toml
@@ -1,4 +1,15 @@
-[cases.bench_superblocks_found]
+[cases.bench_superblocks_format]
+code = '''
+    lfs_t lfs;
+
+    BENCH_START();
+    lfs_format(&lfs, cfg) => 0;
+    BENCH_STOP();
+'''
+
+[cases.bench_superblocks_mount]
+defines.KNOWN_BLOCK_SIZE = [true, false]
+defines.KNOWN_BLOCK_COUNT = [true, false]
 # support benchmarking with files
 defines.N = [0, 1024]
 defines.FILE_SIZE = 8
@@ -28,6 +39,18 @@ code = '''
     }
     lfs_unmount(&lfs) => 0;
 
+    if (KNOWN_BLOCK_SIZE) {
+        cfg->block_size = BLOCK_SIZE;
+    } else {
+        cfg->block_size = 0;
+    }
+
+    if (KNOWN_BLOCK_COUNT) {
+        cfg->block_count = BLOCK_COUNT;
+    } else {
+        cfg->block_count = 0;
+    }
+
     BENCH_START();
     lfs_mount(&lfs, cfg) => 0;
     BENCH_STOP();
@@ -35,22 +58,27 @@ code = '''
     lfs_unmount(&lfs) => 0;
 '''
 
-[cases.bench_superblocks_missing]
+[cases.bench_superblocks_mount_missing]
+defines.KNOWN_BLOCK_SIZE = [true, false]
+defines.KNOWN_BLOCK_COUNT = [true, false]
 code = '''
     lfs_t lfs;
 
-    BENCH_START();
-    int err = lfs_mount(&lfs, cfg);
-    assert(err != 0);
-    BENCH_STOP();
-'''
+    if (KNOWN_BLOCK_SIZE) {
+        cfg->block_size = BLOCK_SIZE;
+    } else {
+        cfg->block_size = 0;
+    }
 
-[cases.bench_superblocks_format]
-code = '''
-    lfs_t lfs;
+    if (KNOWN_BLOCK_COUNT) {
+        cfg->block_count = BLOCK_COUNT;
+    } else {
+        cfg->block_count = 0;
+    }
 
     BENCH_START();
-    lfs_format(&lfs, cfg) => 0;
+    int err = lfs_mount(&lfs, cfg);
+    assert(err != 0);
     BENCH_STOP();
 '''