--- mono-0.97/mono/interpreter/interp.c.orig	2004-06-26 17:55:52.739575943 +0200
+++ mono-0.97/mono/interpreter/interp.c	2004-06-26 17:54:44.489967101 +0200
@@ -1164,7 +1164,7 @@
 		}
 	}
 
-	if (method->klass->valuetype)
+	if (method->klass->valuetype && obj)
 		/* Unbox the instance, since valuetype methods expect an interior pointer. */
 		obj = mono_object_unbox (obj);