diff --git a/chain/src/chain.rs b/chain/src/chain.rs
index 1189ba3e2..9f6121b9b 100644
--- a/chain/src/chain.rs
+++ b/chain/src/chain.rs
@@ -544,9 +544,15 @@ impl Chain {
 		let mut txhashset =
 			txhashset::TxHashSet::open(self.db_root.clone(), self.store.clone(), Some(&header))?;
 
+		// validate against a read-only extension first (some of the validation
+		// runs additional rewinds)
+		txhashset::extending_readonly(&mut txhashset, |extension| {
+			extension.rewind(&header, &header)?;
+			extension.validate(&header, false, status)
+		})?;
+
 		// all good, prepare a new batch and update all the required records
 		let mut batch = self.store.batch()?;
-		// Note: we are validating against a writeable extension.
 		txhashset::extending(&mut txhashset, &mut batch, |extension| {
 			// TODO do we need to rewind here? We have no blocks to rewind
 			// (and we need them for the pos to unremove)
diff --git a/chain/src/txhashset.rs b/chain/src/txhashset.rs
index 4bc853dfa..cbe9166ee 100644
--- a/chain/src/txhashset.rs
+++ b/chain/src/txhashset.rs
@@ -911,8 +911,7 @@ impl<'a> Extension<'a> {
 
 		// Verify kernel roots for all past headers, need to be last as it rewinds
 		// a lot without resetting
-		// TODO broken in fast sync, fix
-		// self.verify_kernel_history(header)?;
+		self.verify_kernel_history(header)?;
 
 		Ok((output_sum, kernel_sum))
 	}
@@ -1047,7 +1046,6 @@ impl<'a> Extension<'a> {
 		// header, rewind and check each root. This fixes a potential weakness in
 		// fast sync where a reorg past the horizon could allow a whole rewrite of
 		// the kernel set.
-		let header_head = header.clone();
 		let mut current = header.clone();
 		let empty_bitmap = Bitmap::create();
 		loop {