diff --git a/Lib/ruby/director.swg b/Lib/ruby/director.swg
index ad41abfae..aba5b7cec 100644
--- a/Lib/ruby/director.swg
+++ b/Lib/ruby/director.swg
@@ -11,6 +11,7 @@
 
 #ifdef __cplusplus
 
+#include <assert.h>
 #include <string>
 
 struct swig_body_args