diff --git a/lfs.c b/lfs.c
index b2c0a631aac..463c097a298 100644
--- a/lfs.c
+++ b/lfs.c
@@ -975,6 +975,7 @@ static int lfs_index_find(lfs_t *lfs,
             return err;
         }
 
+        assert(head >= 2 && head <= lfs->cfg->block_count);
         current -= 1 << skip;
     }
 
@@ -1054,6 +1055,8 @@ static int lfs_index_extend(lfs_t *lfs,
                     return err;
                 }
             }
+
+            assert(head >= 2 && head <= lfs->cfg->block_count);
         }
 
         *off = 4*skips;
@@ -1428,18 +1431,17 @@ lfs_ssize_t lfs_file_write(lfs_t *lfs, lfs_file_t *file,
         // check if we need a new block
         if (!(file->flags & LFS_F_WRITING) ||
                 file->off == lfs->cfg->block_size) {
-            if (!(file->flags & LFS_F_WRITING)) {
+            if (!(file->flags & LFS_F_WRITING) && file->pos > 0) {
                 // find out which block we're extending from
                 int err = lfs_index_find(lfs, &file->cache, NULL,
                         file->head, file->size,
-                        file->pos, &file->block, &file->off);
+                        file->pos-1, &file->block, &file->off);
                 if (err) {
                     return err;
                 }
 
                 // mark cache as dirty since we may have read data into it
                 file->cache.block = 0xffffffff;
-                file->flags |= LFS_F_WRITING;
             }
 
             // extend file with new blocks
@@ -1450,6 +1452,8 @@ lfs_ssize_t lfs_file_write(lfs_t *lfs, lfs_file_t *file,
             if (err) {
                 return err;
             }
+
+            file->flags |= LFS_F_WRITING;
         }
 
         // program as much as we can in current block
diff --git a/tests/test_seek.sh b/tests/test_seek.sh
index 8c0093852eb..1f70ee391cd 100755
--- a/tests/test_seek.sh
+++ b/tests/test_seek.sh
@@ -277,5 +277,33 @@ tests/test.py << TEST
     lfs_unmount(&lfs) => 0;
 TEST
 
+echo "--- Boundary seek and write ---"
+tests/test.py << TEST
+    lfs_mount(&lfs, &cfg) => 0;
+    lfs_file_open(&lfs, &file[0], "hello/kitty42", LFS_O_RDWR) => 0;
+
+    size = strlen("hedgehoghog");
+    const lfs_soff_t offsets[] = {512, 1020, 513, 1021, 511, 1019};
+
+    for (int i = 0; i < sizeof(offsets) / sizeof(offsets[0]); i++) {
+        lfs_soff_t off = offsets[i];
+        memcpy(buffer, "hedgehoghog", size);
+        lfs_file_seek(&lfs, &file[0], off, LFS_SEEK_SET) >= 0 => 1;
+        lfs_file_write(&lfs, &file[0], buffer, size) => size;
+        lfs_file_seek(&lfs, &file[0], off, LFS_SEEK_SET) => off+size;
+        lfs_file_read(&lfs, &file[0], buffer, size) => size;
+        memcmp(buffer, "hedgehoghog", size) => 0;
+
+        lfs_file_seek(&lfs, &file[0], 0, LFS_SEEK_SET) => off+size;
+        lfs_file_read(&lfs, &file[0], buffer, size) => size;
+        memcmp(buffer, "kittycatcat", size) => 0;
+
+        lfs_file_sync(&lfs, &file[0]) => 0;
+    }
+
+    lfs_file_close(&lfs, &file[0]) => 0;
+    lfs_unmount(&lfs) => 0;
+TEST
+
 echo "--- Results ---"
 tests/stats.py