diff --git a/packer/plugin/builder.go b/packer/plugin/builder.go
index b8999ff94..a0a89d57b 100644
--- a/packer/plugin/builder.go
+++ b/packer/plugin/builder.go
@@ -10,7 +10,7 @@ type cmdBuilder struct {
 	client  *Client
 }
 
-func (b *cmdBuilder) Prepare(config ...interface{}) error {
+func (b *cmdBuilder) Prepare(config ...interface{}) ([]string, error) {
 	defer func() {
 		r := recover()
 		b.checkExit(r, nil)
diff --git a/packer/plugin/builder_test.go b/packer/plugin/builder_test.go
index e3356f50f..23a80999f 100644
--- a/packer/plugin/builder_test.go
+++ b/packer/plugin/builder_test.go
@@ -1,23 +1,10 @@
 package plugin
 
 import (
-	"github.com/mitchellh/packer/packer"
 	"os/exec"
 	"testing"
 )
 
-type helperBuilder byte
-
-func (helperBuilder) Prepare(...interface{}) error {
-	return nil
-}
-
-func (helperBuilder) Run(packer.Ui, packer.Hook, packer.Cache) (packer.Artifact, error) {
-	return nil, nil
-}
-
-func (helperBuilder) Cancel() {}
-
 func TestBuilder_NoExist(t *testing.T) {
 	c := NewClient(&ClientConfig{Cmd: exec.Command("i-should-not-exist")})
 	defer c.Kill()
diff --git a/packer/plugin/plugin_test.go b/packer/plugin/plugin_test.go
index 10c3f9d5c..d80aceaf7 100644
--- a/packer/plugin/plugin_test.go
+++ b/packer/plugin/plugin_test.go
@@ -54,7 +54,7 @@ func TestHelperProcess(*testing.T) {
 		fmt.Printf("%s1|:1234\n", APIVersion)
 		<-make(chan int)
 	case "builder":
-		ServeBuilder(new(helperBuilder))
+		ServeBuilder(new(packer.MockBuilder))
 	case "command":
 		ServeCommand(new(helperCommand))
 	case "hook":