diff --git a/builder/vsphere/iso/builder.go b/builder/vsphere/iso/builder.go
index 0cae03611..7c809a746 100644
--- a/builder/vsphere/iso/builder.go
+++ b/builder/vsphere/iso/builder.go
@@ -19,11 +19,10 @@ type Builder struct {
 func (b *Builder) ConfigSpec() hcldec.ObjectSpec { return b.config.FlatMapstructure().HCL2Spec() }
 
 func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
-	c, warnings, errs := NewConfig(raws...)
+	warnings, errs := b.config.Prepare()
 	if errs != nil {
 		return warnings, nil, errs
 	}
-	b.config = c
 
 	return warnings, nil, nil
 }
diff --git a/builder/vsphere/iso/config.go b/builder/vsphere/iso/config.go
index e5c060519..dd04c8052 100644
--- a/builder/vsphere/iso/config.go
+++ b/builder/vsphere/iso/config.go
@@ -38,8 +38,7 @@ type Config struct {
 	ctx interpolate.Context
 }
 
-func NewConfig(raws ...interface{}) (*Config, []string, error) {
-	c := new(Config)
+func (c *Config) Prepare(raws ...interface{}) ([]string, error) {
 	err := config.Decode(c, &config.DecodeOpts{
 		Interpolate:        true,
 		InterpolateContext: &c.ctx,
@@ -50,7 +49,7 @@ func NewConfig(raws ...interface{}) (*Config, []string, error) {
 		},
 	}, raws...)
 	if err != nil {
-		return nil, nil, err
+		return nil, err
 	}
 
 	warnings := make([]string, 0)
@@ -75,8 +74,8 @@ func NewConfig(raws ...interface{}) (*Config, []string, error) {
 	errs = packer.MultiErrorAppend(errs, c.ShutdownConfig.Prepare()...)
 
 	if len(errs.Errors) > 0 {
-		return nil, nil, errs
+		return nil, errs
 	}
 
-	return c, nil, nil
+	return nil, nil
 }