diff --git a/convert.go b/convert.go
index de9d4d8..3448ae2 100644
--- a/convert.go
+++ b/convert.go
@@ -28,7 +28,14 @@ func UpgradeV0toV1(path string, prefixLen int) error {
 }
 
 func DowngradeV1toV0(path string) error {
-	err := os.Remove(filepath.Join(path, SHARDING_FN))
+	fun, err := ReadShardFunc(path)
+	if err != nil {
+		return err
+	} else if fun.funName != "prefix" {
+		return fmt.Errorf("%s: can only downgrade datastore that use the 'prefix' sharding function", path)
+	}
+
+	err = os.Remove(filepath.Join(path, SHARDING_FN))
 	if err != nil {
 		return err
 	}
diff --git a/convert_test.go b/convert_test.go
index f41d220..d8d4e4e 100644
--- a/convert_test.go
+++ b/convert_test.go
@@ -96,7 +96,7 @@ func TestMoveRestart(t *testing.T) {
 
 	// there should be nothing left in the new datastore
 	rmEmptyDatastore(t, v2dir)
-	
+
 	// try the move again, again should fail
 	createDatastore(t, v2dir, flatfs.NextToLast(2))
 	err = flatfs.Move(v1dir, v2dir, nil)
@@ -139,12 +139,12 @@ func TestUpgradeDownload(t *testing.T) {
 	keys, blocks := populateDatastore(t, tempdir)
 	checkKeys(t, tempdir, keys, blocks)
 
-	//err := flatfs.UpgradeV0toV1(tempdir, 3)
-	//if err == nil {
-	//	t.Fatalf("UpgradeV0toV1 on already v1 should fail.")
-	//}
+	err := flatfs.UpgradeV0toV1(tempdir, 3)
+	if err == nil {
+		t.Fatalf("UpgradeV0toV1 on already v1 should fail.")
+	}
 
-	err := flatfs.DowngradeV1toV0(tempdir)
+	err = flatfs.DowngradeV1toV0(tempdir)
 	if err != nil {
 		t.Fatalf("DowngradeV1toV0 fail: %v\n", err)
 	}
@@ -165,6 +165,18 @@ func TestUpgradeDownload(t *testing.T) {
 	checkKeys(t, tempdir, keys, blocks)
 }
 
+func TestDownloadNonPrefix(t *testing.T) {
+	tempdir, cleanup := tempdir(t)
+	defer cleanup()
+
+	createDatastore(t, tempdir, flatfs.NextToLast(2))
+
+	err := flatfs.DowngradeV1toV0(tempdir)
+	if err == nil {
+		t.Fatalf("DowngradeV1toV0 should have failed", err)
+	}
+}
+
 func createDatastore(t *testing.T, dir string, fun *flatfs.ShardIdV1) {
 	err := flatfs.Create(dir, fun)
 	if err != nil {
diff --git a/shard.go b/shard.go
index 8ee4729..acfa450 100644
--- a/shard.go
+++ b/shard.go
@@ -121,7 +121,7 @@ func ReadShardFunc(dir string) (*ShardIdV1, error) {
 }
 
 func WriteShardFunc(dir string, id *ShardIdV1) error {
-	file, err := os.Create(filepath.Join(dir, SHARDING_FN))
+	file, err := os.OpenFile(filepath.Join(dir, SHARDING_FN), os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0666)
 	if err != nil {
 		return err
 	}