diff --git a/builder.go b/builder.go
index f771e33ffe..cae8b55827 100644
--- a/builder.go
+++ b/builder.go
@@ -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")
 	}
 
diff --git a/graph.go b/graph.go
index 21d9d9407c..9a8fd1b3cb 100644
--- a/graph.go
+++ b/graph.go
@@ -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
diff --git a/runtime.go b/runtime.go
index 5958aa1811..7db3b40325 100644
--- a/runtime.go
+++ b/runtime.go
@@ -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)
diff --git a/runtime_test.go b/runtime_test.go
index 1ef92688c6..64956baa67 100644
--- a/runtime_test.go
+++ b/runtime_test.go
@@ -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) {
diff --git a/utils.go b/utils.go
index a4ddb98eb6..4b416cd1e6 100644
--- a/utils.go
+++ b/utils.go
@@ -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.