diff --git a/terraform/eval_context_builtin.go b/terraform/eval_context_builtin.go
index 7e1eac310a..d355e36ef1 100644
--- a/terraform/eval_context_builtin.go
+++ b/terraform/eval_context_builtin.go
@@ -29,7 +29,8 @@ type BuiltinEvalContext struct {
 	StateValue          *State
 	StateLock           *sync.RWMutex
 
-	once sync.Once
+	once    sync.Once
+	varLock sync.Mutex
 }
 
 func (ctx *BuiltinEvalContext) Hook(fn func(Hook) (HookAction, error)) error {
@@ -238,6 +239,9 @@ func (ctx *BuiltinEvalContext) Path() []string {
 }
 
 func (ctx *BuiltinEvalContext) SetVariables(vs map[string]string) {
+	ctx.varLock.Lock()
+	defer ctx.varLock.Unlock()
+
 	for k, v := range vs {
 		ctx.Interpolater.Variables[k] = v
 	}
diff --git a/terraform/graph_config_node_variable.go b/terraform/graph_config_node_variable.go
index 964b7b0067..46aeff4a35 100644
--- a/terraform/graph_config_node_variable.go
+++ b/terraform/graph_config_node_variable.go
@@ -66,3 +66,33 @@ func (n *GraphNodeConfigVariable) SetVariableValue(v *config.RawConfig) {
 func (n *GraphNodeConfigVariable) Proxy() bool {
 	return true
 }
+
+// GraphNodeEvalable impl.
+func (n *GraphNodeConfigVariable) EvalTree() EvalNode {
+	// If we have no value, do nothing
+	if n.Value == nil {
+		return &EvalNoop{}
+	}
+
+	// Otherwise, interpolate the value of this variable and set it
+	// within the variables mapping.
+	var config *ResourceConfig
+	variables := make(map[string]string)
+	return &EvalSequence{
+		Nodes: []EvalNode{
+			&EvalInterpolate{
+				Config: n.Value,
+				Output: &config,
+			},
+
+			&EvalVariableBlock{
+				Config:    &config,
+				Variables: variables,
+			},
+
+			&EvalSetVariables{
+				Variables: variables,
+			},
+		},
+	}
+}