diff --git a/eth/main.cpp b/eth/main.cpp
index 3324afcdd..5f2349ae4 100644
--- a/eth/main.cpp
+++ b/eth/main.cpp
@@ -787,11 +787,14 @@ int main(int argc, char** argv)
 		else if (arg == "-C" || arg == "--cpu")
 			minerType = MinerType::CPU;
 		else if (arg == "-G" || arg == "--opencl")
+		{
 			minerType = MinerType::GPU;
+			miningThreads = 1;
 		/*<< "    -s,--import-secret <secret>  Import a secret key into the key store and use as the default." << endl
 		<< "    -S,--import-session-secret <secret>  Import a secret key into the key store and use as the default for this session only." << endl
 		<< "    --sign-key <address>  Sign all transactions with the key of the given address." << endl
 		<< "    --session-sign-key <address>  Sign all transactions with the key of the given address for this session only." << endl*/
+		}
 		else if ((arg == "-s" || arg == "--import-secret") && i + 1 < argc)
 		{
 			Secret s(fromHex(argv[++i]));
diff --git a/ethminer/main.cpp b/ethminer/main.cpp
index 97e125e65..10d8436f0 100644
--- a/ethminer/main.cpp
+++ b/ethminer/main.cpp
@@ -398,7 +398,10 @@ int main(int argc, char** argv)
 		else if (arg == "-C" || arg == "--cpu")
 			minerType = MinerType::CPU;
 		else if (arg == "-G" || arg == "--opencl")
+		{
 			minerType = MinerType::GPU;
+			miningThreads = 1;
+		}
 		else if ((arg == "-D" || arg == "--create-dag") && i + 1 < argc)
 		{
 			string m = boost::to_lower_copy(string(argv[++i]));
diff --git a/libethcore/EthashAux.cpp b/libethcore/EthashAux.cpp
index 9bb2c4aa9..59155610f 100644
--- a/libethcore/EthashAux.cpp
+++ b/libethcore/EthashAux.cpp
@@ -116,6 +116,8 @@ EthashAux::LightAllocation::LightAllocation(h256 const& _seedHash)
 {
 	uint64_t blockNumber = EthashAux::number(_seedHash);
 	light = ethash_light_new(blockNumber);
+	if (!light)
+		BOOST_THROW_EXCEPTION(ExternalFunctionFailure("ethash_light_new()"));
 	size = ethash_get_cachesize(blockNumber);
 }
 
@@ -132,6 +134,8 @@ bytesConstRef EthashAux::LightAllocation::data() const
 EthashAux::FullAllocation::FullAllocation(ethash_light_t _light, ethash_callback_t _cb)
 {
 	full = ethash_full_new(_light, _cb);
+	if (!full)
+		BOOST_THROW_EXCEPTION(ExternalFunctionFailure("ethash_full_new()"));
 }
 
 EthashAux::FullAllocation::~FullAllocation()