diff --git a/Design-Doc%3A-%60level%60-tool.md b/Design-Doc%3A-%60level%60-tool.md
index bf29331..1e9b01c 100644
--- a/Design-Doc%3A-%60level%60-tool.md
+++ b/Design-Doc%3A-%60level%60-tool.md
@@ -22,7 +22,7 @@ The string property `reskit-layer` set on a Tile Layer indicates which layer sho
 * `a`/`A` for Plane A
 * `b`/`B` for Plane B
 
-Tile layers without `reskit-layer` set are ignored by the `level` tool.
+Tile layers without `reskit-layer` set are ignored by the `level` tool. If no tile layer is specified using the `reskit-layer` property, or you use a setting invalid for the selected system, you will receive an error on the command line.
 
 ## Collision Map
 The boolean property `reskit-collision` set on a Tile Layer to `true` indicates which layer should be used for building the tile collision map. Any non-zero item placed indicates a tile which the player is not permitted to pass through. How you implement this is up to your game engine - reskit will simply save the collision map in a `.bin` file as an artifact (`collision.bin`).