diff --git a/ethcore/src/ethereum/ethash.rs b/ethcore/src/ethereum/ethash.rs
index 746da9069..c8b8d7b31 100644
--- a/ethcore/src/ethereum/ethash.rs
+++ b/ethcore/src/ethereum/ethash.rs
@@ -255,12 +255,21 @@ impl Ethash {
 
 	/// Convert an Ethash boundary to its original difficulty. Basically just `f(x) = 2^256 / x`.
 	pub fn boundary_to_difficulty(boundary: &H256) -> U256 {
-		U256::from((U512::one() << 256) / U256::from(boundary.as_slice()).into())
+		let d = U256::from(*boundary);
+		if d <= U256::one() {
+			U256::max_value()
+		} else {
+			((U256::one() << 255) / d) << 1
+		}
 	}
 
 	/// Convert an Ethash difficulty to the target boundary. Basically just `f(x) = 2^256 / x`.
 	pub fn difficulty_to_boundary(difficulty: &U256) -> H256 {
-		U256::from((U512::one() << 256) / difficulty.into()).into()
+		if *difficulty <= U256::one() {
+			U256::max_value().into()
+		} else {
+			(((U256::one() << 255) / *difficulty) << 1).into()
+		}
 	}
 
 	fn to_ethash(hash: H256) -> EH256 {
@@ -291,12 +300,11 @@ impl Header {
 
 #[cfg(test)]
 mod tests {
-	extern crate ethash;
-
 	use common::*;
 	use block::*;
 	use tests::helpers::*;
 	use super::super::new_morden;
+	use super::Ethash;
 
 	#[test]
 	fn on_close_block() {
@@ -509,5 +517,15 @@ mod tests {
 		}
 	}
 
+	#[test]
+	fn test_difficulty_to_boundary() {
+		// result of f(0) is undefined, so do not assert the result
+		let _ = Ethash::difficulty_to_boundary(&U256::from(0));
+		assert_eq!(Ethash::difficulty_to_boundary(&U256::from(1)), H256::from(U256::max_value()));
+		assert_eq!(Ethash::difficulty_to_boundary(&U256::from(2)), H256::from_str("8000000000000000000000000000000000000000000000000000000000000000").unwrap());
+		assert_eq!(Ethash::difficulty_to_boundary(&U256::from(4)), H256::from_str("4000000000000000000000000000000000000000000000000000000000000000").unwrap());
+		assert_eq!(Ethash::difficulty_to_boundary(&U256::from(32)), H256::from_str("0800000000000000000000000000000000000000000000000000000000000000").unwrap());
+	}
+
 	// TODO: difficulty test
 }