@@ -72,7 +72,7 @@ func (builder *Builder) Create(config *Config) (*Container, error) {

 		builder.mergeConfig(config, img.Config)

 	}

-	if config.Cmd == nil {

+	if config.Cmd == nil || len(config.Cmd) == 0 {

 		return nil, fmt.Errorf("No command specified")

 	}

@@ -253,14 +253,14 @@ func (graph *Graph) WalkAll(handler func(*Image)) error {

 func (graph *Graph) ByParent() (map[string][]*Image, error) {

 	byParent := make(map[string][]*Image)

 	err := graph.WalkAll(func(image *Image) {

-		image, err := graph.Get(image.Parent)

+		parent, err := graph.Get(image.Parent)

 		if err != nil {

 			return

 		}

-		if children, exists := byParent[image.Parent]; exists {

-			byParent[image.Parent] = []*Image{image}

+		if children, exists := byParent[parent.Id]; exists {

+			byParent[parent.Id] = []*Image{image}

 		} else {

-			byParent[image.Parent] = append(children, image)

+			byParent[parent.Id] = append(children, image)

 		}

 	})

 	return byParent, err

@@ -178,6 +178,10 @@ func (runtime *Runtime) LogToDisk(src *writeBroadcaster, dst string) error {

 }

 func (runtime *Runtime) Destroy(container *Container) error {

+	if container == nil {

+		return fmt.Errorf("The given container is <nil>")

+	}

+

 	element := runtime.getContainerElement(container.Id)

 	if element == nil {

 		return fmt.Errorf("Container %v not found - maybe it was already destroyed?", container.Id)

@@ -118,7 +118,10 @@ func TestRuntimeCreate(t *testing.T) {

 	if len(runtime.List()) != 0 {

 		t.Errorf("Expected 0 containers, %v found", len(runtime.List()))

 	}

-	container, err := NewBuilder(runtime).Create(&Config{

+

+	builder := NewBuilder(runtime)

+

+	container, err := builder.Create(&Config{

 		Image: GetTestImage(runtime).Id,

 		Cmd:   []string{"ls", "-al"},

 	},

@@ -157,6 +160,26 @@ func TestRuntimeCreate(t *testing.T) {

 	if !runtime.Exists(container.Id) {

 		t.Errorf("Exists() returned false for a newly created container")

 	}

+

+	// Make sure crete with bad parameters returns an error

+	_, err = builder.Create(

+		&Config{

+			Image: GetTestImage(runtime).Id,

+		},

+	)

+	if err == nil {

+		t.Fatal("Builder.Create should throw an error when Cmd is missing")

+	}

+

+	_, err = builder.Create(

+		&Config{

+			Image: GetTestImage(runtime).Id,

+			Cmd:   []string{},

+		},

+	)

+	if err == nil {

+		t.Fatal("Builder.Create should throw an error when Cmd is empty")

+	}

 }

 func TestDestroy(t *testing.T) {

@@ -442,7 +442,11 @@ func GetKernelVersion() (*KernelVersionInfo, error) {

 }

 func (k *KernelVersionInfo) String() string {

-	return fmt.Sprintf("%d.%d.%d-%s", k.Kernel, k.Major, k.Minor, k.Flavor)

+	flavor := ""

+	if len(k.Flavor) > 0 {

+		flavor = fmt.Sprintf("-%s", k.Flavor)

+	}

+	return fmt.Sprintf("%d.%d.%d%s", k.Kernel, k.Major, k.Minor, flavor)

 }

 // Compare two KernelVersionInfo struct.