diff --git a/pow/ethash.go b/pow/ethash.go
index 1e577a587..9adc38540 100644
--- a/pow/ethash.go
+++ b/pow/ethash.go
@@ -428,7 +428,7 @@ func (ethash *Ethash) cache(block uint64) []uint32 {
 	current, future := ethash.caches[epoch], (*cache)(nil)
 	if current == nil {
 		// No in-memory cache, evict the oldest if the cache limit was reached
-		for len(ethash.caches) >= ethash.cachesinmem {
+		for len(ethash.caches) > 0 && len(ethash.caches) >= ethash.cachesinmem {
 			var evict *cache
 			for _, cache := range ethash.caches {
 				if evict == nil || evict.used.After(cache.used) {
@@ -480,22 +480,16 @@ func (ethash *Ethash) cache(block uint64) []uint32 {
 // Search implements PoW, attempting to find a nonce that satisfies the block's
 // difficulty requirements.
 func (ethash *Ethash) Search(block Block, stop <-chan struct{}) (uint64, []byte) {
-	// Extract some data from the block
-	var (
-		hash   = block.HashNoNonce().Bytes()
-		diff   = block.Difficulty()
-		target = new(big.Int).Div(maxUint256, diff)
-	)
-	// Retrieve the mining dataset
-	dataset, size := ethash.dataset(block.NumberU64()), datasetSize(block.NumberU64())
-
-	// Start generating random nonces until we abort or find a good one
 	var (
+		hash     = block.HashNoNonce().Bytes()
+		diff     = block.Difficulty()
+		target   = new(big.Int).Div(maxUint256, diff)
+		dataset  = ethash.dataset(block.NumberU64())
+		rand     = rand.New(rand.NewSource(time.Now().UnixNano()))
+		nonce    = uint64(rand.Int63())
 		attempts int64
-
-		rand  = rand.New(rand.NewSource(time.Now().UnixNano()))
-		nonce = uint64(rand.Int63())
 	)
+	// Start generating random nonces until we abort or find a good one
 	for {
 		select {
 		case <-stop:
@@ -511,7 +505,7 @@ func (ethash *Ethash) Search(block Block, stop <-chan struct{}) (uint64, []byte)
 				attempts = 0
 			}
 			// Compute the PoW value of this nonce
-			digest, result := hashimotoFull(size, dataset, hash, nonce)
+			digest, result := hashimotoFull(dataset, hash, nonce)
 			if new(big.Int).SetBytes(result).Cmp(target) <= 0 {
 				return nonce, digest
 			}
@@ -532,7 +526,7 @@ func (ethash *Ethash) dataset(block uint64) []uint32 {
 	current, future := ethash.datasets[epoch], (*dataset)(nil)
 	if current == nil {
 		// No in-memory dataset, evict the oldest if the dataset limit was reached
-		for len(ethash.datasets) >= ethash.dagsinmem {
+		for len(ethash.datasets) > 0 && len(ethash.datasets) >= ethash.dagsinmem {
 			var evict *dataset
 			for _, dataset := range ethash.datasets {
 				if evict == nil || evict.used.After(dataset.used) {
diff --git a/pow/ethash_algo.go b/pow/ethash_algo.go
index 3737cc5d7..1e996785f 100644
--- a/pow/ethash_algo.go
+++ b/pow/ethash_algo.go
@@ -349,12 +349,12 @@ func hashimotoLight(size uint64, cache []uint32, hash []byte, nonce uint64) ([]b
 // hashimotoFull aggregates data from the full dataset (using the full in-memory
 // dataset) in order to produce our final value for a particular header hash and
 // nonce.
-func hashimotoFull(size uint64, dataset []uint32, hash []byte, nonce uint64) ([]byte, []byte) {
+func hashimotoFull(dataset []uint32, hash []byte, nonce uint64) ([]byte, []byte) {
 	lookup := func(index uint32) []uint32 {
 		offset := index * hashWords
 		return dataset[offset : offset+hashWords]
 	}
-	return hashimoto(hash, nonce, size, lookup)
+	return hashimoto(hash, nonce, uint64(len(dataset))*4, lookup)
 }
 
 // datasetSizes is a lookup table for the ethash dataset size for the first 2048
diff --git a/pow/ethash_algo_test.go b/pow/ethash_algo_test.go
index c881874ff..0605d70ad 100644
--- a/pow/ethash_algo_test.go
+++ b/pow/ethash_algo_test.go
@@ -660,7 +660,7 @@ func TestHashimoto(t *testing.T) {
 	if !bytes.Equal(result, wantResult) {
 		t.Errorf("light hashimoto result mismatch: have %x, want %x", result, wantResult)
 	}
-	digest, result = hashimotoFull(32*1024, dataset, hash, nonce)
+	digest, result = hashimotoFull(dataset, hash, nonce)
 	if !bytes.Equal(digest, wantDigest) {
 		t.Errorf("full hashimoto digest mismatch: have %x, want %x", digest, wantDigest)
 	}
@@ -713,6 +713,17 @@ func TestConcurrentDiskCacheGeneration(t *testing.T) {
 	pend.Wait()
 }
 
+func TestTestMode(t *testing.T) {
+	head := &types.Header{Difficulty: big.NewInt(100)}
+	ethash := NewTestEthash()
+	nonce, mix := ethash.Search(types.NewBlockWithHeader(head), nil)
+	head.Nonce = types.EncodeNonce(nonce)
+	copy(head.MixDigest[:], mix)
+	if err := ethash.Verify(types.NewBlockWithHeader(head)); err != nil {
+		t.Error("unexpected Verify error:", err)
+	}
+}
+
 // Benchmarks the cache generation performance.
 func BenchmarkCacheGeneration(b *testing.B) {
 	for i := 0; i < b.N; i++ {
@@ -758,6 +769,6 @@ func BenchmarkHashimotoFullSmall(b *testing.B) {
 
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
-		hashimotoFull(32*65536, dataset, hash, 0)
+		hashimotoFull(dataset, hash, 0)
 	}
 }