diff --git a/src/hb-mutex.hh b/src/hb-mutex.hh
index 6914b2245..053f9ddcc 100644
--- a/src/hb-mutex.hh
+++ b/src/hb-mutex.hh
@@ -108,10 +108,11 @@ struct hb_mutex_t
 
 struct hb_lock_t
 {
-  hb_lock_t (hb_mutex_t &mutex_) : mutex (mutex_) { mutex.lock (); }
-  ~hb_lock_t () { mutex.unlock (); }
+  hb_lock_t (hb_mutex_t &mutex_) : mutex (&mutex_) { mutex->lock (); }
+  hb_lock_t (hb_mutex_t *mutex_) : mutex (mutex_) { if (mutex) mutex->lock (); }
+  ~hb_lock_t () { if (mutex) mutex->unlock (); }
   private:
-  hb_mutex_t &mutex;
+  hb_mutex_t *mutex;
 };
 
 
diff --git a/src/hb-subset-accelerator.hh b/src/hb-subset-accelerator.hh
index 3158ad76c..37c7fa137 100644
--- a/src/hb-subset-accelerator.hh
+++ b/src/hb-subset-accelerator.hh
@@ -75,17 +75,25 @@ struct hb_subset_accelerator_t
     hb_free (accel);
   }
 
-  hb_subset_accelerator_t(const hb_map_t& unicode_to_gid_,
+  hb_subset_accelerator_t (const hb_map_t& unicode_to_gid_,
                           const hb_set_t& unicodes_)
       : unicode_to_gid(unicode_to_gid_), unicodes(unicodes_),
         cmap_cache(nullptr), destroy_cmap_cache(nullptr),
         has_seac(false), cff_accelerator(nullptr), destroy_cff_accelerator(nullptr)
+  { sanitized_table_cache_lock.init (); }
 
-  {}
+  ~hb_subset_accelerator_t ()
+  { sanitized_table_cache_lock.fini (); }
 
   // Generic
+
+  mutable hb_mutex_t sanitized_table_cache_lock;
+  mutable hb_hashmap_t<hb_tag_t, hb::unique_ptr<hb_blob_t>> sanitized_table_cache;
+
   const hb_map_t unicode_to_gid;
   const hb_set_t unicodes;
+
+  // cmap
   OT::SubtableUnicodesCache* cmap_cache;
   hb_destroy_func_t destroy_cmap_cache;
 
diff --git a/src/hb-subset-plan.hh b/src/hb-subset-plan.hh
index e6cccac40..1dea7aad9 100644
--- a/src/hb-subset-plan.hh
+++ b/src/hb-subset-plan.hh
@@ -219,18 +219,20 @@ struct hb_subset_plan_t
   template<typename T>
   hb_blob_ptr_t<T> source_table()
   {
-    if (sanitized_table_cache
-        && !sanitized_table_cache->in_error ()
-        && sanitized_table_cache->has (+T::tableTag)) {
-      return hb_blob_reference (sanitized_table_cache->get (+T::tableTag).get ());
+    hb_lock_t (accelerator ? &accelerator->sanitized_table_cache_lock : nullptr);
+
+    auto *cache = accelerator ? &accelerator->sanitized_table_cache : sanitized_table_cache;
+    if (cache
+        && !cache->in_error ()
+        && cache->has (+T::tableTag)) {
+      return hb_blob_reference (cache->get (+T::tableTag).get ());
     }
 
     hb::unique_ptr<hb_blob_t> table_blob {hb_sanitize_context_t ().reference_table<T> (source)};
     hb_blob_t* ret = hb_blob_reference (table_blob.get ());
 
-    if (likely (sanitized_table_cache))
-      sanitized_table_cache->set (+T::tableTag,
-                                  std::move (table_blob));
+    if (likely (cache))
+      cache->set (+T::tableTag, std::move (table_blob));
 
     return ret;
   }