diff --git a/libethcore/BlockInfo.cpp b/libethcore/BlockInfo.cpp
index 71f5469dd..53ff29626 100644
--- a/libethcore/BlockInfo.cpp
+++ b/libethcore/BlockInfo.cpp
@@ -209,7 +209,7 @@ u256 BlockInfo::calculateDifficulty(BlockInfo const& _parent) const
 	u256 o = max<u256>(c_minimumDifficulty, m_timestamp >= _parent.m_timestamp + c_durationLimit ? _parent.m_difficulty - (_parent.m_difficulty / c_difficultyBoundDivisor) : (_parent.m_difficulty + (_parent.m_difficulty / c_difficultyBoundDivisor)));
 	unsigned periodCount = unsigned(_parent.number() + 1) / c_expDiffPeriod;
 	if (periodCount > 2)
-		o = (u256)max<bigint>(c_minimumDifficulty, bigint(o) - (bigint(1) << (periodCount - 2)));	// latter will eventually become negative, so ensure it's a bigint.
+		o = max<u256>(c_minimumDifficulty, o + (u256(1) << (periodCount - 2)));	// latter will eventually become huge, so ensure it's a bigint.
 	return o;
 }