diff --git a/src/hb-ot-shape-fallback.cc b/src/hb-ot-shape-fallback.cc
index ea8312b22..4da53f4cc 100644
--- a/src/hb-ot-shape-fallback.cc
+++ b/src/hb-ot-shape-fallback.cc
@@ -307,6 +307,9 @@ position_around_base (const hb_ot_shape_plan_t *plan,
 		      unsigned int end)
 {
   hb_direction_t horiz_dir = HB_DIRECTION_INVALID;
+
+  buffer->unsafe_to_break (base, end);
+
   hb_glyph_extents_t base_extents;
   if (!font->get_glyph_extents (buffer->info[base].codepoint,
 				&base_extents))