diff --git a/grid.c b/grid.c
index 2ffae9a3..a3f77dba 100644
--- a/grid.c
+++ b/grid.c
@@ -12,12 +12,13 @@ grid_swap_row(struct grid *grid, int row_a, int row_b)
 {
     assert(grid->offset >= 0);
     assert(row_a != row_b);
-    assert(row_a >= 0);
-    assert(row_b >= 0);
 
     int real_a = (grid->offset + row_a + grid->num_rows) % grid->num_rows;
     int real_b = (grid->offset + row_b + grid->num_rows) % grid->num_rows;
 
+    assert(real_a >= 0);
+    assert(real_b >= 0);
+
     struct row *tmp = grid->rows[real_a];
     grid->rows[real_a] = grid->rows[real_b];
     grid->rows[real_b] = tmp;