diff --git a/dag/dag.go b/dag/dag.go
index 1a76dc1cf..b58b40717 100644
--- a/dag/dag.go
+++ b/dag/dag.go
@@ -24,6 +24,10 @@ type WalkFunc func(Vertex) error
 // walk as an argument
 type DepthWalkFunc func(Vertex, int) error
 
+func (g *AcyclicGraph) DirectedGraph() Grapher {
+	return g
+}
+
 // Returns a Set that includes every Vertex yielded by walking down from the
 // provided starting Vertex v.
 func (g *AcyclicGraph) Ancestors(v Vertex) (*Set, error) {
diff --git a/dag/graph.go b/dag/graph.go
index e713ff3ef..456eb0826 100644
--- a/dag/graph.go
+++ b/dag/graph.go
@@ -17,6 +17,18 @@ type Graph struct {
 	once      sync.Once
 }
 
+// Subgrapher allows a Vertex to be a Graph itself, by returning a Grapher.
+type Subgrapher interface {
+	Subgraph() Grapher
+}
+
+// A Grapher is any type that returns a Grapher, mainly used to identify
+// dag.Graph and dag.AcyclicGraph.  In the case of Graph and AcyclicGraph, they
+// return themselves.
+type Grapher interface {
+	DirectedGraph() Grapher
+}
+
 // Vertex of the graph.
 type Vertex interface{}
 
@@ -27,6 +39,10 @@ type NamedVertex interface {
 	Name() string
 }
 
+func (g *Graph) DirectedGraph() Grapher {
+	return g
+}
+
 // Vertices returns the list of all the vertices in the graph.
 func (g *Graph) Vertices() []Vertex {
 	list := g.vertices.List()
diff --git a/dag/marshal.go b/dag/marshal.go
index 7b0f1e8e3..c44f0a4ce 100644
--- a/dag/marshal.go
+++ b/dag/marshal.go
@@ -60,7 +60,7 @@ func newMarshalGraph(name string, g *Graph) *marshalGraph {
 
 	for _, v := range g.Vertices() {
 		id := marshalVertexID(v)
-		if sg, ok := marshalSubgraph(v); ok {
+		if sg, ok := marshalSubgrapher(v); ok {
 
 			sdg := newMarshalGraph(VertexName(v), sg)
 			sdg.ID = id
@@ -129,22 +129,19 @@ func marshalVertexID(v Vertex) string {
 	panic("unhashable value in graph")
 }
 
-func debugSubgraph(v Vertex) (*Graph, bool) {
-	val := reflect.ValueOf(v)
-	m, ok := val.Type().MethodByName("Subgraph")
+// check for a Subgrapher, and return the underlying *Graph.
+func marshalSubgrapher(v Vertex) (*Graph, bool) {
+	sg, ok := v.(Subgrapher)
 	if !ok {
 		return nil, false
 	}
 
-	if m.Type.NumOut() != 1 {
-		return nil, false
+	switch g := sg.Subgraph().DirectedGraph().(type) {
+	case *Graph:
+		return g, true
+	case *AcyclicGraph:
+		return &g.Graph, true
 	}
 
-	// can't check for the subgraph type, because we can't import terraform, so
-	// we assume this is the correct method.
-	// TODO: create a dag interface type that we can satisfy
-
-	sg := val.MethodByName("Subgraph").Call(nil)[0]
-	ag := sg.Elem().FieldByName("AcyclicGraph").Interface().(AcyclicGraph)
-	return &ag.Graph, true
+	return nil, false
 }
diff --git a/terraform/graph.go b/terraform/graph.go
index 5f114a5c4..5174ef925 100644
--- a/terraform/graph.go
+++ b/terraform/graph.go
@@ -43,6 +43,10 @@ type Graph struct {
 	once sync.Once
 }
 
+func (g *Graph) DirectedGraph() dag.Grapher {
+	return &g.AcyclicGraph
+}
+
 // Annotations returns the annotations that are configured for the
 // given vertex. The map is guaranteed to be non-nil but may be empty.
 //
@@ -317,7 +321,7 @@ func (g *Graph) walk(walker GraphWalker) error {
 			walker.Debug().Printf(
 				"[DEBUG] vertex %T(%s.%s): subgraph\n", v, path, dag.VertexName(v))
 
-			if rerr = sn.Subgraph().walk(walker); rerr != nil {
+			if rerr = sn.Subgraph().(*Graph).walk(walker); rerr != nil {
 				return
 			}
 		}
diff --git a/terraform/graph_config_node_module.go b/terraform/graph_config_node_module.go
index 4396c2e65..5ba36252b 100644
--- a/terraform/graph_config_node_module.go
+++ b/terraform/graph_config_node_module.go
@@ -156,7 +156,7 @@ func (n *graphNodeModuleExpanded) EvalTree() EvalNode {
 
 // GraphNodeFlattenable impl.
 func (n *graphNodeModuleExpanded) FlattenGraph() *Graph {
-	graph := n.Subgraph()
+	graph := n.Subgraph().(*Graph)
 	input := n.Original.Module.RawConfig
 
 	// Go over each vertex and do some modifications to the graph for
@@ -189,7 +189,7 @@ func (n *graphNodeModuleExpanded) FlattenGraph() *Graph {
 }
 
 // GraphNodeSubgraph impl.
-func (n *graphNodeModuleExpanded) Subgraph() *Graph {
+func (n *graphNodeModuleExpanded) Subgraph() dag.Grapher {
 	return n.Graph
 }
 
diff --git a/terraform/graph_config_node_module_test.go b/terraform/graph_config_node_module_test.go
index 6ba015a45..6aeacf788 100644
--- a/terraform/graph_config_node_module_test.go
+++ b/terraform/graph_config_node_module_test.go
@@ -33,7 +33,7 @@ func TestGraphNodeConfigModuleExpand(t *testing.T) {
 		t.Fatalf("err: %s", err)
 	}
 
-	actual := strings.TrimSpace(g.Subgraph().String())
+	actual := strings.TrimSpace(g.Subgraph().(*Graph).String())
 	expected := strings.TrimSpace(testGraphNodeModuleExpandStr)
 	if actual != expected {
 		t.Fatalf("bad:\n\n%s", actual)
diff --git a/terraform/graph_debug.go b/terraform/graph_debug.go
index 80c84e780..181f5f4bf 100644
--- a/terraform/graph_debug.go
+++ b/terraform/graph_debug.go
@@ -184,7 +184,7 @@ func (dg *DebugGraph) buildSubgraph(modName string, g *Graph, modDepth int) erro
 
 	for _, v := range g.Vertices() {
 		if sn, ok := v.(GraphNodeSubgraph); ok {
-			subgraphVertices[v] = sn.Subgraph()
+			subgraphVertices[v] = sn.Subgraph().(*Graph)
 		}
 	}
 
@@ -200,7 +200,7 @@ func (dg *DebugGraph) buildSubgraph(modName string, g *Graph, modDepth int) erro
 		toDraw = append(toDraw, v)
 
 		if sn, ok := v.(GraphNodeSubgraph); ok {
-			subgraphVertices[v] = sn.Subgraph()
+			subgraphVertices[v] = sn.Subgraph().(*Graph)
 		}
 		return nil
 	}
diff --git a/terraform/graph_dot_test.go b/terraform/graph_dot_test.go
index cc4376e46..164327b8e 100644
--- a/terraform/graph_dot_test.go
+++ b/terraform/graph_dot_test.go
@@ -299,7 +299,7 @@ type testDrawableSubgraph struct {
 func (node *testDrawableSubgraph) Name() string {
 	return node.VertexName
 }
-func (node *testDrawableSubgraph) Subgraph() *Graph {
+func (node *testDrawableSubgraph) Subgraph() dag.Grapher {
 	return node.SubgraphMock
 }
 func (node *testDrawableSubgraph) DotNode(n string, opts *dag.DotOpts) *dot.Node {
diff --git a/terraform/transform_expand.go b/terraform/transform_expand.go
index e5120119a..b58c6bf17 100644
--- a/terraform/transform_expand.go
+++ b/terraform/transform_expand.go
@@ -24,7 +24,7 @@ type GraphNodeDynamicExpandable interface {
 // GraphNodeSubgraph is an interface a node can implement if it has
 // a larger subgraph that should be walked.
 type GraphNodeSubgraph interface {
-	Subgraph() *Graph
+	Subgraph() dag.Grapher
 }
 
 // ExpandTransform is a transformer that does a subgraph expansion
@@ -56,7 +56,7 @@ func (n *GraphNodeBasicSubgraph) Name() string {
 	return n.NameValue
 }
 
-func (n *GraphNodeBasicSubgraph) Subgraph() *Graph {
+func (n *GraphNodeBasicSubgraph) Subgraph() dag.Grapher {
 	return n.Graph
 }
 
diff --git a/terraform/transform_expand_test.go b/terraform/transform_expand_test.go
index e0ee9cca2..d422eddf0 100644
--- a/terraform/transform_expand_test.go
+++ b/terraform/transform_expand_test.go
@@ -30,7 +30,7 @@ func TestExpandTransform(t *testing.T) {
 		t.Fatalf("not subgraph: %#v", out)
 	}
 
-	actual := strings.TrimSpace(sn.Subgraph().String())
+	actual := strings.TrimSpace(sn.Subgraph().(*Graph).String())
 	expected := strings.TrimSpace(testExpandTransformStr)
 	if actual != expected {
 		t.Fatalf("bad: %s", actual)
@@ -66,7 +66,7 @@ type testSubgraph struct {
 	Graph *Graph
 }
 
-func (n *testSubgraph) Subgraph() *Graph {
+func (n *testSubgraph) Subgraph() dag.Grapher {
 	return n.Graph
 }