diff --git a/simgear/scene/model/model.cxx b/simgear/scene/model/model.cxx
index 66de79b5..be5d9f48 100644
--- a/simgear/scene/model/model.cxx
+++ b/simgear/scene/model/model.cxx
@@ -43,7 +43,7 @@ model_filter = true;
 static int
 model_filter_callback (ssgEntity * entity, int mask)
 {
-  return model_filter;
+  return model_filter ? 1 : 0;
 }
 
 /**