diff --git a/sixel.c b/sixel.c
index 4de17c11..8c95e70b 100644
--- a/sixel.c
+++ b/sixel.c
@@ -372,23 +372,26 @@ _sixel_overwrite_by_rectangle(
     const int start = row;
     const int end = row + height - 1;
 
+    /* We should never generate scrollback wrapping sixels */
+    assert(end < term->grid->num_rows);
+
     const int scrollback_rel_start = rebase_row(term, start);
 
     tll_foreach(term->grid->sixel_images, it) {
         struct sixel *six = &it->item;
 
         const int six_start = six->pos.row;
-        const int six_end = (six_start + six->rows - 1) & (term->grid->num_rows - 1);
+        const int six_end = (six_start + six->rows - 1);
         const int six_scrollback_rel_end = rebase_row(term, six_end);
 
+        /* We should never generate scrollback wrapping sixels */
+        assert(six_end < term->grid->num_rows);
+
         if (six_scrollback_rel_end < scrollback_rel_start) {
             /* All remaining sixels are *before* our rectangle */
             break;
         }
 
-        /* We should never generate scrollback wrapping sixels */
-        assert(six_end >= six_start);
-
         if ((start <= six_start && end >= six_start) ||  /* Crosses sixel start boundary */
             (start <= six_end && end >= six_end) ||      /* Crosses sixel end boundary */
             (start >= six_start && end <= six_end))      /* Fully within sixel range */