diff --git a/caching/superblock.cc b/caching/superblock.cc
index 5281afb..a76d8b0 100644
--- a/caching/superblock.cc
+++ b/caching/superblock.cc
@@ -253,7 +253,9 @@ superblock_traits::pack(superblock const &sb, superblock_disk &disk)
 	disk.write_hits = to_disk<le32>(core.write_hits);
 	disk.write_misses = to_disk<le32>(core.write_misses);
 
-	if (core.version >= 2)
+	// The version may be overridden, meaning the dirty root may not
+	// actually be present.
+	if (core.version >= 2 && core.dirty_root)
 		disk.dirty_root = to_disk<le64>(*core.dirty_root);
 }