diff --git a/terraform/ui_output_mock.go b/terraform/ui_output_mock.go
index 7852bc423..d828c921c 100644
--- a/terraform/ui_output_mock.go
+++ b/terraform/ui_output_mock.go
@@ -1,13 +1,18 @@
 package terraform
 
+import "sync"
+
 // MockUIOutput is an implementation of UIOutput that can be used for tests.
 type MockUIOutput struct {
+	sync.Mutex
 	OutputCalled  bool
 	OutputMessage string
 	OutputFn      func(string)
 }
 
 func (o *MockUIOutput) Output(v string) {
+	o.Lock()
+	defer o.Unlock()
 	o.OutputCalled = true
 	o.OutputMessage = v
 	if o.OutputFn != nil {