diff --git a/techtree-manager/src/tree.rs b/techtree-manager/src/tree.rs
index fef523d..db6a93f 100644
--- a/techtree-manager/src/tree.rs
+++ b/techtree-manager/src/tree.rs
@@ -2,6 +2,8 @@ use std::collections::BTreeMap;
 
 use petgraph::visit::EdgeRef as _;
 
+const HASH_EPOCH: u32 = 0x00000001;
+
 /// Element in the techtree
 #[derive(Debug, Clone, Hash, PartialEq, Eq, serde::Serialize)]
 pub struct Element {
@@ -420,7 +422,7 @@ impl<'a> Subtree<'a> {
             .collect();
         edges.sort();
 
-        let json_data = serde_json::ser::to_string(&(nodes, edges)).unwrap();
+        let json_data = serde_json::ser::to_string(&(nodes, edges, HASH_EPOCH)).unwrap();
         sha256::digest(json_data)
     }
 }