diff --git a/types.h b/types.h
index 3594cc6..5d86132 100644
--- a/types.h
+++ b/types.h
@@ -256,7 +256,7 @@ typedef struct InputFrame
   bool dobuild;
   enum BoxType build_type;
   enum CompassRotation build_rotation;
-} InputFrame;
+} InputFrame; // the length of this is assumed to be constant
 
 typedef struct PlatonicDetection
 {