diff --git a/index.js b/index.js
index ba5ee5fc..e02a15c4 100644
--- a/index.js
+++ b/index.js
@@ -535,7 +535,7 @@ class Hypercore extends EventEmitter {
 
   get fork () {
     if (this.opened === false) return 0
-    return this.core.tree.fork
+    return this.state.tree.fork
   }
 
   get peers () {
diff --git a/lib/core.js b/lib/core.js
index acb954cb..7234071f 100644
--- a/lib/core.js
+++ b/lib/core.js
@@ -406,7 +406,7 @@ module.exports = class Core {
         signature = this.verifier.sign(state.tree.batch(), keyPair)
       }
 
-      const tree = await this.state._overwrite(state, length, treeLength, signature, false, atom)
+      const tree = await this.state._overwrite(state, this.tree.fork, length, treeLength, signature, false, atom)
 
       // gc blocks from source
       if (treeLength < length) {
diff --git a/lib/session-state.js b/lib/session-state.js
index 8699c84b..80add55e 100644
--- a/lib/session-state.js
+++ b/lib/session-state.js
@@ -404,7 +404,7 @@ module.exports = class SessionState {
     }
   }
 
-  async _overwrite (source, length, treeLength, signature, isDependent, atom) {
+  async _overwrite (source, fork, length, treeLength, signature, isDependent, atom) {
     const blockPromises = []
     const treePromises = []
     const rootPromises = []
@@ -481,21 +481,27 @@ module.exports = class SessionState {
     }
 
     const tree = {
-      fork: this.tree.fork,
+      fork,
       length,
       rootHash: crypto.tree(roots),
       signature
     }
 
-    const upgraded = treeLength < this.tree.length || this.tree.length < length
+    const upgraded = treeLength < this.tree.length || this.tree.length < length || tree.fork !== this.tree.fork
 
-    if (upgraded) writer.setCoreHead(tree)
+    if (upgraded) {
+      writer.setCoreHead(tree)
+    }
 
     const dependency = isDependent ? updateDependency(this, length) : null
 
     await this.flushWriteBatch()
 
-    if (upgraded) this.tree.setRoots(roots, signature)
+    if (upgraded) {
+      this.tree.fork = fork
+      this.tree.setRoots(roots, signature)
+    }
+
     if (dependency) this.storage.updateDependencies(dependency.length)
 
     return tree
@@ -509,8 +515,9 @@ module.exports = class SessionState {
 
     try {
       const origLength = this.tree.length
+      const fork = treeLength < origLength ? this.tree.fork + 1 : this.tree.fork
 
-      const tree = await this._overwrite(state, length, treeLength, null, state === this.core.state, atom)
+      const tree = await this._overwrite(state, fork, length, treeLength, null, state === this.core.state, atom)
 
       const bitfield = { start: treeLength, length: tree.length - treeLength, drop: false }