diff --git a/openbox/stacking.h b/openbox/stacking.h
index afa4a6c6..ded4644e 100644
--- a/openbox/stacking.h
+++ b/openbox/stacking.h
@@ -27,6 +27,7 @@
 
 /*! The possible stacking layers a client window can be a part of */
 typedef enum {
+    OB_STACKING_LAYER_INVALID,
     OB_STACKING_LAYER_DESKTOP,     /*!< 0 - desktop windows */
     OB_STACKING_LAYER_BELOW,       /*!< 1 - normal windows w/ below */
     OB_STACKING_LAYER_NORMAL,      /*!< 2 - normal windows */