diff --git a/src/tetris.c b/src/tetris.c
index f826798..f6ab3ae 100644
--- a/src/tetris.c
+++ b/src/tetris.c
@@ -74,7 +74,8 @@ void tetris_place(u8 *restrict *restrict rows, u8 tetromino, uint x, uint y) {
 	uint *pos = (uint[8]){x, y, x, y, x, y, x, y};
 	uint *end = pos + 8;
 	tetris_get_blocks(tetromino, pos);
-	tetromino &= 7; /* strip rotation data */
+	tetromino &= 7;    /* strip rotation data */
+	tetromino |= 0x80; /* set an undefined bit, to signify there is data here. */
 	for (; pos < end; pos += 2)
 		rows[*(pos + 1)][*pos] = tetromino;
 }