diff --git a/glib/tests/gvariant.c b/glib/tests/gvariant.c
index e215e9861..ebdd559c3 100644
--- a/glib/tests/gvariant.c
+++ b/glib/tests/gvariant.c
@@ -2325,10 +2325,19 @@ test_byteswap (void)
    * This might not work because it’s not necessarily possible to make an
    * arbitrary random variant non-normal. Adding a single zero byte to the end
    * often makes something non-normal but still readable. */
-  three_size_copy = three.size + 1;
-  three_data_copy = g_malloc (three_size_copy);
-  memcpy (three_data_copy, three.data, three.size);
-  three_data_copy[three.size] = '\0';
+  if (three.data != NULL)
+    {
+      g_assert_cmpuint (three.size, !=, 0);
+
+      three_size_copy = three.size + 1;
+      three_data_copy = g_malloc (three_size_copy);
+      memcpy (three_data_copy, three.data, three.size);
+      three_data_copy[three.size] = '\0';
+    }
+  else
+    {
+      g_assert_cmpuint (three.size, ==, 0);
+    }
 
   three_variant = g_variant_new_from_data (G_VARIANT_TYPE (g_variant_type_info_get_type_string (three.type_info)),
                                            three_data_copy, three_size_copy, FALSE, NULL, NULL);