diff --git a/src/hb-ot-layout-gpos-table.hh b/src/hb-ot-layout-gpos-table.hh
index 0b828e8ad..738ca8c44 100644
--- a/src/hb-ot-layout-gpos-table.hh
+++ b/src/hb-ot-layout-gpos-table.hh
@@ -432,6 +432,7 @@ struct MarkArray : ArrayOf<MarkRecord>	/* Array of MarkRecords--in Coverage orde
 
     hb_position_t mark_x, mark_y, base_x, base_y;
 
+    buffer->unsafe_to_break (glyph_pos, buffer->idx);
     mark_anchor.get_anchor (c, buffer->cur().codepoint, &mark_x, &mark_y);
     glyph_anchor.get_anchor (c, buffer->info[glyph_pos].codepoint, &base_x, &base_y);