diff --git a/frontend/apps/reader/modules/readerhighlight.lua b/frontend/apps/reader/modules/readerhighlight.lua
index 082a1a359..37cbaa992 100644
--- a/frontend/apps/reader/modules/readerhighlight.lua
+++ b/frontend/apps/reader/modules/readerhighlight.lua
@@ -1854,6 +1854,8 @@ function ReaderHighlight:onHoldPan(_, ges)
     local old_text = self.selected_text and self.selected_text.text
     self.selected_text = self.ui.document:getTextFromPositions(self.hold_pos, self.holdpan_pos)
     if self.selected_text and self.selected_text.sboxes and #self.selected_text.sboxes == 0 then
+        -- abort highlighting if crengine doesn't provide sboxes for current positions
+        -- may happen in TXT files with disabled txt_preformatted
         self:clear()
         return true
     end