diff --git a/.travis.yml b/.travis.yml
index 0651efc..b6c4f7a 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -9,6 +9,9 @@ script:
         -include stdio.h -Werror' make all size
 
     # run tests
-    - CFLAGS="-DLFS_READ_SIZE=16  -DLFS_PROG_SIZE=16"  make test
+    - make test
+
+    # run tests with a few different configurations
     - CFLAGS="-DLFS_READ_SIZE=1   -DLFS_PROG_SIZE=1"   make test
     - CFLAGS="-DLFS_READ_SIZE=512 -DLFS_PROG_SIZE=512" make test
+    - CFLAGS="-DLFS_BLOCK_COUNT=1023" make test
diff --git a/lfs.c b/lfs.c
index 463c097..76a1b19 100644
--- a/lfs.c
+++ b/lfs.c
@@ -262,7 +262,10 @@ int lfs_deorphan(lfs_t *lfs);
 static int lfs_alloc_lookahead(void *p, lfs_block_t block) {
     lfs_t *lfs = p;
 
-    lfs_block_t off = (block - lfs->free.start) % lfs->cfg->block_count;
+    lfs_block_t off = (((lfs_soff_t)(block - lfs->free.start)
+                % (lfs_soff_t)(lfs->cfg->block_count))
+            + lfs->cfg->block_count) % lfs->cfg->block_count;
+
     if (off < lfs->cfg->lookahead) {
         lfs->free.lookahead[off / 32] |= 1U << (off % 32);
     }
@@ -994,6 +997,7 @@ static int lfs_index_extend(lfs_t *lfs,
         if (err) {
             return err;
         }
+        assert(*block >= 2 && *block <= lfs->cfg->block_count);
 
         err = lfs_bd_erase(lfs, *block);
         if (err) {