diff --git a/ethcore/light/src/cht.rs b/ethcore/light/src/cht.rs
index 537a3ed6a..13edadee1 100644
--- a/ethcore/light/src/cht.rs
+++ b/ethcore/light/src/cht.rs
@@ -17,11 +17,38 @@
 pub const SIZE: u64 = 2048;
 
 /// Convert a block number to a CHT number.
-pub fn block_to_cht_number(block_num: u64) -> u64 {
-	(block_num + 1) / SIZE
+/// Returns `None` for `block_num` == 0, `Some` otherwise.
+pub fn block_to_cht_number(block_num: u64) -> Option<u64> {
+	match block_num {
+		0 => None,
+		n => Some((n - 1) / SIZE),
+	}
 }
 
 /// Get the starting block of a given CHT.
+/// CHT 0 includes block 1...SIZE,
+/// CHT 1 includes block SIZE + 1 ... 2*SIZE
+/// More generally: CHT N includes block (1 + N*SIZE)...((N+1)*SIZE).
+/// This is because the genesis hash is assumed to be known
+/// and including it would be redundant.
 pub fn start_number(cht_num: u64) -> u64 {
 	(cht_num * SIZE) + 1
 }
+
+#[cfg(test)]
+mod tests {
+	#[test]
+	fn block_to_cht_number() {
+		assert!(::cht::block_to_cht_number(0).is_none());
+		assert_eq!(::cht::block_to_cht_number(1).unwrap(), 0);
+		assert_eq!(::cht::block_to_cht_number(::cht::SIZE + 1).unwrap(), 1);
+		assert_eq!(::cht::block_to_cht_number(::cht::SIZE).unwrap(), 0);
+	}
+
+	#[test]
+	fn start_number() {
+		assert_eq!(::cht::start_number(0), 1);
+		assert_eq!(::cht::start_number(1), ::cht::SIZE + 1);
+		assert_eq!(::cht::start_number(2), ::cht::SIZE * 2 + 1);
+	}
+}
diff --git a/ethcore/light/src/on_demand/mod.rs b/ethcore/light/src/on_demand/mod.rs
index dfc78a629..a01de7598 100644
--- a/ethcore/light/src/on_demand/mod.rs
+++ b/ethcore/light/src/on_demand/mod.rs
@@ -41,6 +41,8 @@ pub enum Error {
 	Canceled,
 	/// No suitable peers available to serve the request.
 	NoPeersAvailable,
+	/// Invalid request.
+	InvalidRequest,
 	/// Request timed out.
 	TimedOut,
 }
@@ -112,7 +114,15 @@ impl OnDemand {
 	// dispatch the request, completing the request if no peers available.
 	fn dispatch_header_by_number(&self, ctx: &BasicContext, req: request::HeaderByNumber, sender: Sender<encoded::Header>) {
 		let num = req.num;
-		let cht_num = ::cht::block_to_cht_number(req.num);
+		let cht_num = match ::cht::block_to_cht_number(req.num) {
+			Some(cht_num) => cht_num,
+			None => {
+				warn!(target: "on_demand", "Attempted to dispatch invalid header proof: req.num == 0");
+				sender.complete(Err(Error::InvalidRequest));
+				return;
+			}
+		};
+
 		let les_req = LesRequest::HeaderProofs(les_request::HeaderProofs {
 			requests: vec![les_request::HeaderProof {
 				cht_number: cht_num,
diff --git a/ethcore/light/src/provider.rs b/ethcore/light/src/provider.rs
index 14f8a148d..a2ee36f82 100644
--- a/ethcore/light/src/provider.rs
+++ b/ethcore/light/src/provider.rs
@@ -230,7 +230,7 @@ impl<T: ProvingBlockChainClient + ?Sized> Provider for T {
 		use util::MemoryDB;
 		use util::trie::{Trie, TrieMut, TrieDB, TrieDBMut, Recorder};
 
-		if req.cht_number != ::cht::block_to_cht_number(req.block_number) {
+		if Some(req.cht_number) != ::cht::block_to_cht_number(req.block_number) {
 			debug!(target: "les_provider", "Requested CHT number mismatch with block number.");
 			return None;
 		}
@@ -257,7 +257,7 @@ impl<T: ProvingBlockChainClient + ?Sized> Provider for T {
 		}
 		let needed_hdr = needed_hdr.expect("`needed_hdr` always set in loop, number checked before; qed");
 
-		let mut recorder = Recorder::new();
+		let mut recorder = Recorder::with_depth(req.from_level);
 		let t = TrieDB::new(&memdb, &root)
 			.expect("Same DB and root as just produced by TrieDBMut; qed");
 
@@ -275,3 +275,27 @@ impl<T: ProvingBlockChainClient + ?Sized> Provider for T {
 		BlockChainClient::ready_transactions(self)
 	}
 }
+
+#[cfg(test)]
+mod tests {
+	use ethcore::client::{EachBlockWith, TestBlockChainClient};
+	use super::Provider;
+
+	#[test]
+	fn cht_proof() {
+		let client = TestBlockChainClient::new();
+		client.add_blocks(2000, EachBlockWith::Nothing);
+
+		let req = ::request::HeaderProof {
+			cht_number: 0,
+			block_number: 1500,
+			from_level: 0,
+		};
+
+		assert!(client.header_proof(req.clone()).is_none());
+
+		client.add_blocks(48, EachBlockWith::Nothing);
+
+		assert!(client.header_proof(req.clone()).is_some());
+	}
+}
diff --git a/ethcore/light/src/types/les_request.rs b/ethcore/light/src/types/les_request.rs
index dde8ac2f8..6dced2731 100644
--- a/ethcore/light/src/types/les_request.rs
+++ b/ethcore/light/src/types/les_request.rs
@@ -120,7 +120,7 @@ pub struct ContractCodes {
 pub struct HeaderProof {
 	/// Number of the CHT.
 	pub cht_number: u64,
-	/// Block number requested.
+	/// Block number requested. May not be 0: genesis isn't included in any CHT.
 	pub block_number: u64,
 	/// If greater than zero, trie nodes beyond this level may be omitted.
 	pub from_level: u32,