diff --git a/src/s3ql/_sqlite3ext.cpp b/src/s3ql/_sqlite3ext.cpp
index ef61f7ee..7fb9245e 100644
--- a/src/s3ql/_sqlite3ext.cpp
+++ b/src/s3ql/_sqlite3ext.cpp
@@ -129,8 +129,14 @@ static int vfstraceWrite(
     if (!p->block_map) {
         return rc;
     }
+
+    size_t end_off = iOfst + iAmt;
+    size_t max_blockno = end_off / blocksize;
+    if (max_blockno * blocksize < end_off) {
+        max_blockno++;
+    }
     for (size_t blockno = iOfst / blocksize;
-        blockno <= (iOfst + iAmt) / blocksize;
+        blockno < max_blockno;
         blockno++) {
         p->block_map->insert(blockno);
     }
diff --git a/tests/t1_database.py b/tests/t1_database.py
index 7f957b9e..0616f979 100755
--- a/tests/t1_database.py
+++ b/tests/t1_database.py
@@ -16,6 +16,8 @@
 
 
 import logging
+import math
+import os
 import tempfile
 from argparse import Namespace
 from typing import List
@@ -66,6 +68,20 @@ def test_track_dirty():
         db.close()
 
 
+def test_track_dirty_count():
+    sqlite3ext.reset()
+    with tempfile.NamedTemporaryFile() as tmpfh:
+        db = Connection(tmpfh.name, BLOCKSIZE)
+        db.execute("CREATE TABLE foo (id INT);")
+        db.execute("INSERT INTO FOO VALUES(42)")
+
+        db.checkpoint()
+        db_size = tmpfh.seek(0, os.SEEK_END)
+        assert db.dirty_blocks.get_count() == math.ceil(db_size / BLOCKSIZE)
+
+        db.close()
+
+
 @pytest.fixture
 def backend():
     with tempfile.TemporaryDirectory(prefix="s3ql-backend-") as backend_dir: