diff --git a/Lib/ruby/rubyhead.swg b/Lib/ruby/rubyhead.swg
index a34f7eab7..90becd694 100644
--- a/Lib/ruby/rubyhead.swg
+++ b/Lib/ruby/rubyhead.swg
@@ -106,3 +106,7 @@ typedef struct {
 #define rb_undef_alloc_func(klass) rb_undef_method(CLASS_OF((klass)), "new")
 #endif
 
+/* Contract support */
+
+#define SWIG_contract_assert(expr, msg) if (!(expr)) { rb_raise(rb_eRuntimeError, (char *) msg ); } else
+