diff --git a/terraform/graph_walk_context.go b/terraform/graph_walk_context.go
index 2994081ff..ac6310d08 100644
--- a/terraform/graph_walk_context.go
+++ b/terraform/graph_walk_context.go
@@ -81,11 +81,12 @@ func (w *ContextGraphWalker) EnterPath(path []string) EvalContext {
 		StateValue:          w.Context.state,
 		StateLock:           &w.Context.stateLock,
 		Interpolater: &Interpolater{
-			Operation: w.Operation,
-			Module:    w.Context.module,
-			State:     w.Context.state,
-			StateLock: &w.Context.stateLock,
-			Variables: variables,
+			Operation:     w.Operation,
+			Module:        w.Context.module,
+			State:         w.Context.state,
+			StateLock:     &w.Context.stateLock,
+			Variables:     variables,
+			VariablesLock: &w.interpolaterVarLock,
 		},
 		InterpolaterVars:    w.interpolaterVars,
 		InterpolaterVarLock: &w.interpolaterVarLock,
diff --git a/terraform/interpolate.go b/terraform/interpolate.go
index 3db060b0b..7ec549c93 100644
--- a/terraform/interpolate.go
+++ b/terraform/interpolate.go
@@ -23,11 +23,12 @@ const (
 // Interpolater is the structure responsible for determining the values
 // for interpolations such as `aws_instance.foo.bar`.
 type Interpolater struct {
-	Operation walkOperation
-	Module    *module.Tree
-	State     *State
-	StateLock *sync.RWMutex
-	Variables map[string]string
+	Operation     walkOperation
+	Module        *module.Tree
+	State         *State
+	StateLock     *sync.RWMutex
+	Variables     map[string]string
+	VariablesLock *sync.Mutex
 }
 
 // InterpolationScope is the current scope of execution. This is required
@@ -273,6 +274,8 @@ func (i *Interpolater) valueUserVar(
 	n string,
 	v *config.UserVariable,
 	result map[string]ast.Variable) error {
+	i.VariablesLock.Lock()
+	defer i.VariablesLock.Unlock()
 	val, ok := i.Variables[v.Name]
 	if ok {
 		result[n] = ast.Variable{