diff --git a/src/game.rs b/src/game.rs
index 7f714e3..b5ef240 100644
--- a/src/game.rs
+++ b/src/game.rs
@@ -360,7 +360,11 @@ impl Game {
     ///      if current tile IDs are [0, 1, 2] the result will be `3`
     #[inline]
     pub fn new_tile_id(&self) -> String {
-        new_unique_id(self.tile_ids())
+        // don't allow `0` - this is a reserved ID
+        // todo new_sprite_id will need to avoid A too
+        let mut ids = self.tile_ids();
+        ids.push("0".to_string());
+        new_unique_id(ids)
     }
 
     /// adds a tile safely and returns the new tile ID
@@ -456,13 +460,16 @@ mod test {
 
     #[test]
     fn test_new_tile_id() {
-        // default tile has an id of 10 ("a"), so 0 is available
-        assert_eq!(crate::mock::game_default().new_tile_id(), "0".to_string());
+        // default tile has an id of 10 ("a"), and 0 is reserved
+        assert_eq!(crate::mock::game_default().new_tile_id(), "1".to_string());
+
+        // for a game with a gap in the tile IDs, check the gap is used
 
         let mut game = crate::mock::game_default();
         let mut tiles: Vec<Tile> = Vec::new();
 
-        for n in 0..9 {
+        // 0 is reserved; upper bound is non-inclusive
+        for n in 1..10 {
             if n != 4 {
                 let mut new_tile = crate::mock::tile_default();
                 new_tile.id = format!("{}", n).to_string();
@@ -474,7 +481,7 @@ mod test {
 
         assert_eq!(game.new_tile_id(), "4".to_string());
 
-        // fill in the space created above, and test that tile IDs get sorted
+        // fill in the space created above, then test that tile IDs get sorted
 
         let mut new_tile = crate::mock::tile_default();
         new_tile.id = "4".to_string();
@@ -487,10 +494,10 @@ mod test {
     fn test_add_tile() {
         let mut game = crate::mock::game_default();
         let new_id = game.add_tile(crate::mock::tile_default());
-        assert_eq!(new_id, "0".to_string());
+        assert_eq!(new_id, "1".to_string());
         assert_eq!(game.tiles.len(), 2);
         let new_id = game.add_tile(crate::mock::tile_default());
-        assert_eq!(new_id, "1".to_string());
+        assert_eq!(new_id, "2".to_string());
         assert_eq!(game.tiles.len(), 3);
     }