diff --git a/piecrust/src/store.rs b/piecrust/src/store.rs
index cee3a81d..3f45a7a9 100644
--- a/piecrust/src/store.rs
+++ b/piecrust/src/store.rs
@@ -478,7 +478,6 @@ impl Commit {
         Some(contract.page_indices.into_iter().map(move |page_index| {
             let tree_opening = self
                 .contracts_merkle
-                .tree
                 .opening(pos)
                 .expect("There must be a leaf for the contract");
 
@@ -520,7 +519,6 @@ impl Commit {
         }
 
         self.contracts_merkle
-            .tree
             .insert(position_from_contract(&contract_id), *element.tree.root());
     }
 
@@ -530,7 +528,7 @@ impl Commit {
     }
 
     pub fn root(&self) -> Ref<Hash> {
-        self.contracts_merkle.tree.root()
+        self.contracts_merkle.root()
     }
 }
 
diff --git a/piecrust/src/store/tree.rs b/piecrust/src/store/tree.rs
index 229fb896..6eecdf47 100644
--- a/piecrust/src/store/tree.rs
+++ b/piecrust/src/store/tree.rs
@@ -148,12 +148,39 @@ pub struct NewContractIndex {
 #[derive(Debug, Clone, Archive, Deserialize, Serialize)]
 #[archive_attr(derive(CheckBytes))]
 pub struct ContractsMerkle {
-    pub tree: Tree,
+    inner_tree: Tree,
+    dict: BTreeMap<u64, u64>,
 }
 
 impl Default for ContractsMerkle {
     fn default() -> Self {
-        Self { tree: Tree::new() }
+        Self {
+            inner_tree: Tree::new(),
+            dict: BTreeMap::new(),
+        }
+    }
+}
+
+impl ContractsMerkle {
+    pub fn insert(&mut self, pos: u64, hash: Hash) {
+        let new_pos = match self.dict.get(&pos) {
+            None => {
+                let new_pos = (self.dict.len() + 1) as u64;
+                self.dict.insert(pos, new_pos);
+                new_pos
+            }
+            Some(p) => *p,
+        };
+        self.inner_tree.insert(new_pos, hash);
+    }
+
+    pub fn opening(&self, pos: u64) -> Option<TreeOpening> {
+        let new_pos = self.dict.get(&pos).expect("pos should exist in dict");
+        self.inner_tree.opening(*new_pos)
+    }
+
+    pub fn root(&self) -> Ref<Hash> {
+        self.inner_tree.root()
     }
 }