diff --git a/builder/hyperv/common/step_clone_vm.go b/builder/hyperv/common/step_clone_vm.go
index 9b005422d..f867bd331 100644
--- a/builder/hyperv/common/step_clone_vm.go
+++ b/builder/hyperv/common/step_clone_vm.go
@@ -40,7 +40,7 @@ func (s *StepCloneVM) Run(state multistep.StateBag) multistep.StepAction {
 	harddrivePath := ""
 	if harddrivePathRaw, ok := state.GetOk("iso_path"); ok {
 		extension := strings.ToLower(filepath.Ext(harddrivePathRaw.(string)))
-		if extension == "vhd" || extension == "vhdx" {
+		if extension == ".vhd" || extension == ".vhdx" {
 			harddrivePath = harddrivePathRaw.(string)
 		} else {
 			log.Println("No existing virtual harddrive, not attaching.")
diff --git a/builder/hyperv/common/step_create_vm.go b/builder/hyperv/common/step_create_vm.go
index 46c4964cb..4b9601842 100644
--- a/builder/hyperv/common/step_create_vm.go
+++ b/builder/hyperv/common/step_create_vm.go
@@ -39,7 +39,7 @@ func (s *StepCreateVM) Run(state multistep.StateBag) multistep.StepAction {
 	harddrivePath := ""
 	if harddrivePathRaw, ok := state.GetOk("iso_path"); ok {
 		extension := strings.ToLower(filepath.Ext(harddrivePathRaw.(string)))
-		if extension == "vhd" || extension == "vhdx" {
+		if extension == ".vhd" || extension == ".vhdx" {
 			harddrivePath = harddrivePathRaw.(string)
 		} else {
 			log.Println("No existing virtual harddrive, not attaching.")
diff --git a/builder/hyperv/iso/builder.go b/builder/hyperv/iso/builder.go
index 382c35394..4d5583239 100644
--- a/builder/hyperv/iso/builder.go
+++ b/builder/hyperv/iso/builder.go
@@ -117,22 +117,15 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	isoWarnings, isoErrs := b.config.ISOConfig.Prepare(&b.config.ctx)
 	warnings = append(warnings, isoWarnings...)
 	errs = packer.MultiErrorAppend(errs, isoErrs...)
-
-	if len(b.config.ISOConfig.ISOUrls) > 0 {
-		extension := strings.ToLower(filepath.Ext(b.config.ISOConfig.ISOUrls[0]))
-		if extension == "vhd" || extension == "vhdx" {
-			b.config.ISOConfig.TargetExtension = extension
-		}
-	}
 	
 	errs = packer.MultiErrorAppend(errs, b.config.FloppyConfig.Prepare(&b.config.ctx)...)
 	errs = packer.MultiErrorAppend(errs, b.config.HTTPConfig.Prepare(&b.config.ctx)...)
 	errs = packer.MultiErrorAppend(errs, b.config.RunConfig.Prepare(&b.config.ctx)...)
 	errs = packer.MultiErrorAppend(errs, b.config.OutputConfig.Prepare(&b.config.ctx, &b.config.PackerConfig)...)
 	errs = packer.MultiErrorAppend(errs, b.config.SSHConfig.Prepare(&b.config.ctx)...)
-	errs = packer.MultiErrorAppend(errs, b.config.ShutdownConfig.Prepare(&b.config.ctx)...)	
+	errs = packer.MultiErrorAppend(errs, b.config.ShutdownConfig.Prepare(&b.config.ctx)...)
 
-	if b.config.ISOConfig.TargetExtension != "vhd" && b.config.ISOConfig.TargetExtension != "vhdx" {
+	if len(b.config.ISOConfig.ISOUrls) < 1 || (strings.ToLower(filepath.Ext(b.config.ISOConfig.ISOUrls[0])) != ".vhd" && strings.ToLower(filepath.Ext(b.config.ISOConfig.ISOUrls[0])) != ".vhdx")  {
 		//We only create a new hard drive if an existing one to copy from does not exist
 		err = b.checkDiskSize()
 		if err != nil {
diff --git a/builder/hyperv/iso/builder_test.go b/builder/hyperv/iso/builder_test.go
index d32ba4f41..0fea9b8e0 100644
--- a/builder/hyperv/iso/builder_test.go
+++ b/builder/hyperv/iso/builder_test.go
@@ -300,6 +300,98 @@ func TestBuilderPrepare_ISOUrl(t *testing.T) {
 	}
 }
 
+func TestBuilderPrepare_SizeNotRequiredWhenUsingExistingHarddrive(t *testing.T) {
+	var b Builder
+	config := testConfig()
+	delete(config, "iso_url")
+	delete(config, "iso_urls")
+	delete(config, "disk_size")
+
+	config["disk_size"] = 1
+
+	// Test just iso_urls set but with vhdx
+	delete(config, "iso_url")
+	config["iso_urls"] = []string{
+		"http://www.packer.io/hdd.vhdx",
+		"http://www.hashicorp.com/dvd.iso",
+	}
+
+	b = Builder{}
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
+	if err != nil {
+		t.Errorf("should not have error: %s", err)
+	}
+
+	expected := []string{
+		"http://www.packer.io/hdd.vhdx",
+		"http://www.hashicorp.com/dvd.iso",
+	}
+	if !reflect.DeepEqual(b.config.ISOUrls, expected) {
+		t.Fatalf("bad: %#v", b.config.ISOUrls)
+	}
+
+	// Test just iso_urls set but with vhd
+	delete(config, "iso_url")
+	config["iso_urls"] = []string{
+		"http://www.packer.io/hdd.vhd",
+		"http://www.hashicorp.com/dvd.iso",
+	}
+
+	b = Builder{}
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
+	if err != nil {
+		t.Errorf("should not have error: %s", err)
+	}
+
+	expected = []string{
+		"http://www.packer.io/hdd.vhd",
+		"http://www.hashicorp.com/dvd.iso",
+	}
+	if !reflect.DeepEqual(b.config.ISOUrls, expected) {
+		t.Fatalf("bad: %#v", b.config.ISOUrls)
+	}
+}
+
+func TestBuilderPrepare_SizeIsRequiredWhenNotUsingExistingHarddrive(t *testing.T) {
+	var b Builder
+	config := testConfig()
+	delete(config, "iso_url")
+	delete(config, "iso_urls")
+	delete(config, "disk_size")
+
+	config["disk_size"] = 1
+
+	// Test just iso_urls set but with vhdx
+	delete(config, "iso_url")
+	config["iso_urls"] = []string{
+		"http://www.packer.io/os.iso",
+		"http://www.hashicorp.com/dvd.iso",
+	}
+
+	b = Builder{}
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
+	if err == nil {
+		t.Errorf("should have error")
+	}
+
+	expected := []string{
+		"http://www.packer.io/os.iso",
+		"http://www.hashicorp.com/dvd.iso",
+	}
+	if !reflect.DeepEqual(b.config.ISOUrls, expected) {
+		t.Fatalf("bad: %#v", b.config.ISOUrls)
+	}
+}
+
 func TestBuilderPrepare_FloppyFiles(t *testing.T) {
 	var b Builder
 	config := testConfig()