diff --git a/doomsday/engine/portable/include/binarytree.h b/doomsday/engine/portable/include/binarytree.h
index a97856841b..f3d70b6b33 100644
--- a/doomsday/engine/portable/include/binarytree.h
+++ b/doomsday/engine/portable/include/binarytree.h
@@ -46,7 +46,7 @@ class BinaryTree
         LEFT  = 1
     };
 
-    static inline void assertValidChildId(ChildId child)
+    static inline void assertValidChildId(ChildId DENG_DEBUG_ONLY(child))
     {
         assert(child == RIGHT || child == LEFT);
     }