diff --git a/libethcore/EthashAux.cpp b/libethcore/EthashAux.cpp
index 06da22f98..4b546404a 100644
--- a/libethcore/EthashAux.cpp
+++ b/libethcore/EthashAux.cpp
@@ -240,8 +240,9 @@ Ethash::Result EthashAux::eval(BlockInfo const& _header, Nonce const& _nonce)
 
 Ethash::Result EthashAux::eval(h256 const& _seedHash, h256 const& _headerHash, Nonce const& _nonce)
 {
-	if (FullType dag = get()->m_fulls[_seedHash].lock())
-		return dag->compute(_headerHash, _nonce);
+	DEV_GUARDED(get()->x_fulls)
+		if (FullType dag = get()->m_fulls[_seedHash].lock())
+			return dag->compute(_headerHash, _nonce);
 	DEV_IF_THROWS(return EthashAux::get()->light(_seedHash)->compute(_headerHash, _nonce))
 	{
 		return Ethash::Result{ ~h256(), h256() };