diff --git a/builder/qemu/builder_test.go b/builder/qemu/builder_test.go
index d0d403666..e726ce456 100644
--- a/builder/qemu/builder_test.go
+++ b/builder/qemu/builder_test.go
@@ -187,6 +187,36 @@ func TestBuilderPrepare_DiskSize(t *testing.T) {
 	}
 }
 
+func TestBuilderPrepare_AdditionalDiskSize(t *testing.T) {
+	var b Builder
+	config := testConfig()
+
+	config["disk_additional_size"] = []string{"1M"}
+	config["disk_image"] = true
+	warns, err := b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
+	if err == nil {
+		t.Fatalf("should have error")
+	}
+
+	delete(config, "disk_image")
+	config["disk_additional_size"] = []string{"1M"}
+	b = Builder{}
+	warns, err = b.Prepare(config)
+	if len(warns) > 0 {
+		t.Fatalf("bad: %#v", warns)
+	}
+	if err != nil {
+		t.Fatalf("should not have error: %s", err)
+	}
+
+	if b.config.AdditionalDiskSize[0] != "1M" {
+		t.Fatalf("bad size: %s", b.config.AdditionalDiskSize)
+	}
+}
+
 func TestBuilderPrepare_Format(t *testing.T) {
 	var b Builder
 	config := testConfig()