diff --git a/simgear/props/condition.hxx b/simgear/props/condition.hxx
index 5f06a5e5..c70e86d7 100644
--- a/simgear/props/condition.hxx
+++ b/simgear/props/condition.hxx
@@ -146,7 +146,7 @@ private:
  * invoke the test() method whenever it needs to decide whether to
  * active itself, draw itself, and so on.
  */
-class SGConditional
+class SGConditional : public SGReferenced
 {
 public:
   SGConditional ();