diff --git a/glib/Value.cs b/glib/Value.cs
index 6a98d2a2f..be45c5ca5 100644
--- a/glib/Value.cs
+++ b/glib/Value.cs
@@ -576,6 +576,10 @@ namespace GLib {
 			if (GType.Is (type, GType.Boxed) && !(val is IWrapper)) {
 				MethodInfo mi = val.GetType ().GetMethod ("Update", BindingFlags.NonPublic | BindingFlags.Instance);
 				IntPtr boxed_ptr = g_value_get_boxed (ref this);
+
+				if (mi == null && !val.GetType ().IsDefined (typeof(StructLayoutAttribute), false))
+					return;
+
 				if (mi == null)
 					Marshal.StructureToPtr (val, boxed_ptr, false);
 				else