diff --git a/src/hb-ot-layout-gsub-table.hh b/src/hb-ot-layout-gsub-table.hh
index 91b898b63af027fcb175cd527107fac3392f5a7c..eef8dc5462a5c234069011717d0d0a7f2303809a 100644
--- a/src/hb-ot-layout-gsub-table.hh
+++ b/src/hb-ot-layout-gsub-table.hh
@@ -47,7 +47,7 @@ struct SingleSubstFormat1
        * https://github.com/harfbuzz/harfbuzz/issues/363 */
       hb_codepoint_t glyph_id = iter.get_glyph ();
       if (c->glyphs->has (glyph_id))
-	c->glyphs->add ((glyph_id + deltaGlyphID) & 0xFFFFu);
+	c->out->add ((glyph_id + deltaGlyphID) & 0xFFFFu);
     }
   }
 
@@ -132,7 +132,7 @@ struct SingleSubstFormat2
       if (unlikely (iter.get_coverage () >= count))
         break; /* Work around malicious fonts. https://github.com/harfbuzz/harfbuzz/issues/363 */
       if (c->glyphs->has (iter.get_glyph ()))
-	c->glyphs->add (substitute[iter.get_coverage ()]);
+	c->out->add (substitute[iter.get_coverage ()]);
     }
   }
 
@@ -263,7 +263,7 @@ struct Sequence
     TRACE_CLOSURE (this);
     unsigned int count = substitute.len;
     for (unsigned int i = 0; i < count; i++)
-      c->glyphs->add (substitute[i]);
+      c->out->add (substitute[i]);
   }
 
   inline void collect_glyphs (hb_collect_glyphs_context_t *c) const
@@ -464,7 +464,7 @@ struct AlternateSubstFormat1
 	const AlternateSet &alt_set = this+alternateSet[iter.get_coverage ()];
 	unsigned int count = alt_set.len;
 	for (unsigned int i = 0; i < count; i++)
-	  c->glyphs->add (alt_set[i]);
+	  c->out->add (alt_set[i]);
       }
     }
   }
@@ -605,7 +605,7 @@ struct Ligature
     for (unsigned int i = 1; i < count; i++)
       if (!c->glyphs->has (component[i]))
         return;
-    c->glyphs->add (ligGlyph);
+    c->out->add (ligGlyph);
   }
 
   inline void collect_glyphs (hb_collect_glyphs_context_t *c) const
@@ -957,7 +957,7 @@ struct ReverseChainSingleSubstFormat1
       if (unlikely (iter.get_coverage () >= count))
         break; /* Work around malicious fonts. https://github.com/harfbuzz/harfbuzz/issues/363 */
       if (c->glyphs->has (iter.get_glyph ()))
-	c->glyphs->add (substitute[iter.get_coverage ()]);
+	c->out->add (substitute[iter.get_coverage ()]);
     }
   }
 
@@ -1163,7 +1163,12 @@ struct SubstLookup : Lookup
       return_trace (HB_VOID);
 
     c->set_recurse_func (dispatch_closure_recurse_func);
-    return_trace (dispatch (c));
+
+    hb_closure_context_t::return_t ret = dispatch (c);
+
+    c->flush ();
+
+    return_trace (ret);
   }
 
   inline hb_collect_glyphs_context_t::return_t collect_glyphs (hb_collect_glyphs_context_t *c) const
@@ -1265,7 +1270,12 @@ struct SubstLookup : Lookup
   {
     if (!c->should_visit_lookup (lookup_index))
       return HB_VOID;
-    return dispatch_recurse_func (c, lookup_index);
+
+    hb_closure_context_t::return_t ret = dispatch_recurse_func (c, lookup_index);
+
+    c->flush ();
+
+    return ret;
   }
 
   template <typename context_t>
diff --git a/src/hb-ot-layout-gsubgpos-private.hh b/src/hb-ot-layout-gsubgpos-private.hh
index aed00f15121f2a5aa426d01f9d21319365b631dc..c03e033567c5b18b94428473dbdc19da2a193284 100644
--- a/src/hb-ot-layout-gsubgpos-private.hh
+++ b/src/hb-ot-layout-gsubgpos-private.hh
@@ -69,12 +69,13 @@ struct hb_closure_context_t :
 
   bool is_lookup_done (unsigned int lookup_index)
   {
-    // Have we visited this lookup with the current set of glyphs?
+    /* Have we visited this lookup with the current set of glyphs? */
     return done_lookups->get (lookup_index) == glyphs->get_population ();
   }
 
   hb_face_t *face;
   hb_set_t *glyphs;
+  hb_set_t *out;
   recurse_func_t recurse_func;
   unsigned int nesting_level_left;
   unsigned int debug_depth;
@@ -85,13 +86,26 @@ struct hb_closure_context_t :
 		        unsigned int nesting_level_left_ = HB_MAX_NESTING_LEVEL) :
 			  face (face_),
 			  glyphs (glyphs_),
+			  out (hb_set_create ()),
 			  recurse_func (nullptr),
 			  nesting_level_left (nesting_level_left_),
 			  debug_depth (0),
                           done_lookups (done_lookups_) {}
 
+  ~hb_closure_context_t (void)
+  {
+    flush ();
+    hb_set_destroy (out);
+  }
+
   void set_recurse_func (recurse_func_t func) { recurse_func = func; }
 
+  void flush (void)
+  {
+    hb_set_union (glyphs, out);
+    hb_set_clear (out);
+  }
+
   private:
   hb_map_t *done_lookups;
 };