diff --git a/dag/dag.go b/dag/dag.go
index c41255d1fc..f2716257b8 100644
--- a/dag/dag.go
+++ b/dag/dag.go
@@ -37,6 +37,26 @@ func (g *AcyclicGraph) Root() (Vertex, error) {
 	return roots[0], nil
 }
 
+// Validate validates the DAG. A DAG is valid if it has a single root
+// with no cycles.
+func (g *AcyclicGraph) Validate() error {
+	if _, err := g.Root(); err != nil {
+		return err
+	}
+
+	var cycles [][]Vertex
+	for _, cycle := range StronglyConnected(&g.Graph) {
+		if len(cycle) > 1 {
+			cycles = append(cycles, cycle)
+		}
+	}
+	if len(cycles) > 0 {
+		return fmt.Errorf("cycles: %#v", cycles)
+	}
+
+	return nil
+}
+
 // Walk walks the graph, calling your callback as each node is visited.
 // This will walk nodes in parallel if it can.
 func (g *AcyclicGraph) Walk(cb WalkFunc) error {
diff --git a/dag/dag_test.go b/dag/dag_test.go
index 2bbea81e41..e607c7ba1b 100644
--- a/dag/dag_test.go
+++ b/dag/dag_test.go
@@ -47,6 +47,34 @@ func TestAcyclicGraphRoot_multiple(t *testing.T) {
 	}
 }
 
+func TestAcyclicGraphValidate(t *testing.T) {
+	var g AcyclicGraph
+	g.Add(1)
+	g.Add(2)
+	g.Add(3)
+	g.Connect(BasicEdge(3, 2))
+	g.Connect(BasicEdge(3, 1))
+
+	if err := g.Validate(); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+}
+
+func TestAcyclicGraphValidate_cycle(t *testing.T) {
+	var g AcyclicGraph
+	g.Add(1)
+	g.Add(2)
+	g.Add(3)
+	g.Connect(BasicEdge(3, 2))
+	g.Connect(BasicEdge(3, 1))
+	g.Connect(BasicEdge(1, 2))
+	g.Connect(BasicEdge(2, 1))
+
+	if err := g.Validate(); err == nil {
+		t.Fatal("should error")
+	}
+}
+
 func TestAcyclicGraphWalk(t *testing.T) {
 	var g AcyclicGraph
 	g.Add(1)
diff --git a/dag/tarjan.go b/dag/tarjan.go
index b5d5b5c8a2..3475dda25c 100644
--- a/dag/tarjan.go
+++ b/dag/tarjan.go
@@ -58,7 +58,7 @@ func strongConnect(g *Graph, v Vertex, data *tarjanData) *tarjanVertex {
 
 	if tv.Lowlink == index {
 		vs := make([]Vertex, 0, 2)
-		for i := len(data.stack) - 1; ; i-- {
+		for i := len(data.stack) - 1; i >= 0; i-- {
 			v := data.stack[i]
 			data.stack[i] = nil
 			data.stack = data.stack[:i]